diff --git a/src/isomorphic/children/__tests__/ReactChildren-test.js b/src/isomorphic/children/__tests__/ReactChildren-test.js
index 9d97007cd1..fc02176a72 100644
--- a/src/isomorphic/children/__tests__/ReactChildren-test.js
+++ b/src/isomorphic/children/__tests__/ReactChildren-test.js
@@ -16,7 +16,6 @@ const ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
 describe('ReactChildren', () => {
   var React;
   var ReactTestUtils;
-  var ReactFeatureFlags;
 
   function normalizeCodeLocInfo(str) {
     return str && str.replace(/at .+?:\d+/g, 'at **');
@@ -883,11 +882,6 @@ describe('ReactChildren', () => {
 
   if (ReactDOMFeatureFlags.useFiber) {
     describe('with fragments enabled', () => {
-      beforeEach(() => {
-        ReactFeatureFlags = require('ReactFeatureFlags');
-        ReactFeatureFlags.disableNewFiberFeatures = false;
-      });
-
       it('warns for keys for arrays of elements in a fragment', () => {
         spyOn(console, 'error');
         class ComponentReturningArray extends React.Component {
diff --git a/src/renderers/__tests__/ReactComponent-test.js b/src/renderers/__tests__/ReactComponent-test.js
index a5f5ce287f..15d55e7826 100644
--- a/src/renderers/__tests__/ReactComponent-test.js
+++ b/src/renderers/__tests__/ReactComponent-test.js
@@ -496,10 +496,6 @@ describe('ReactComponent', () => {
 
   if (ReactDOMFeatureFlags.useFiber) {
     describe('with new features', () => {
-      beforeEach(() => {
-        require('ReactFeatureFlags').disableNewFiberFeatures = false;
-      });
-
       it('warns on function as a return value from a function', () => {
         function Foo() {
           return Foo;
diff --git a/src/renderers/__tests__/ReactEmptyComponent-test.js b/src/renderers/__tests__/ReactEmptyComponent-test.js
index 5368ae06a9..4058e5898a 100644
--- a/src/renderers/__tests__/ReactEmptyComponent-test.js
+++ b/src/renderers/__tests__/ReactEmptyComponent-test.js
@@ -13,10 +13,11 @@
 
 var React;
 var ReactDOM;
-var ReactDOMFeatureFlags;
 var ReactTestUtils;
 var TogglingComponent;
 
+var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
+
 var log;
 
 describe('ReactEmptyComponent', () => {
@@ -25,7 +26,6 @@ describe('ReactEmptyComponent', () => {
 
     React = require('react');
     ReactDOM = require('react-dom');
-    ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
     ReactTestUtils = require('react-dom/test-utils');
 
     log = jasmine.createSpy();
@@ -71,18 +71,20 @@ describe('ReactEmptyComponent', () => {
     expect(container2.children.length).toBe(0);
   });
 
-  it('should still throw when rendering to undefined', () => {
-    class Component extends React.Component {
-      render() {}
-    }
+  if (ReactDOMFeatureFlags.useFiber) {
+    it('should still throw when rendering to undefined', () => {
+      class Component extends React.Component {
+        render() {}
+      }
 
-    expect(function() {
-      ReactTestUtils.renderIntoDocument(<Component />);
-    }).toThrowError(
-      'Component.render(): A valid React element (or null) must be returned. You may ' +
-        'have returned undefined, an array or some other invalid object.',
-    );
-  });
+      expect(function() {
+        ReactTestUtils.renderIntoDocument(<Component />);
+      }).toThrowError(
+        'Component(...): Nothing was returned from render. This usually means a return statement is missing. ' +
+          'Or, to render nothing, return null.',
+      );
+    });
+  }
 
   it('should be able to switch between rendering null and a normal tag', () => {
     var instance1 = (
@@ -232,22 +234,15 @@ describe('ReactEmptyComponent', () => {
   });
 
   it('can render null at the top level', () => {
-    var ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
     var div = document.createElement('div');
-
-    try {
-      if (ReactDOMFeatureFlags.useFiber) {
+    if (ReactDOMFeatureFlags.useFiber) {
+      ReactDOM.render(null, div);
+      expect(div.innerHTML).toBe('');
+    } else {
+      // Stack does not implement this.
+      expect(function() {
         ReactDOM.render(null, div);
-        expect(div.innerHTML).toBe('');
-      } else {
-        // Stack does not implement this.
-        expect(function() {
-          ReactDOM.render(null, div);
-        }).toThrowError('ReactDOM.render(): Invalid component element.');
-      }
-    } finally {
-      ReactFeatureFlags.disableNewFiberFeatures = true;
+      }).toThrowError('ReactDOM.render(): Invalid component element.');
     }
   });
 
diff --git a/src/renderers/__tests__/ReactStatelessComponent-test.js b/src/renderers/__tests__/ReactStatelessComponent-test.js
index f3eafc118e..a1e1a59b7b 100644
--- a/src/renderers/__tests__/ReactStatelessComponent-test.js
+++ b/src/renderers/__tests__/ReactStatelessComponent-test.js
@@ -157,15 +157,17 @@ describe('ReactStatelessComponent', () => {
     });
   }
 
-  it('should throw when stateless component returns undefined', () => {
-    function NotAComponent() {}
-    expect(function() {
-      ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
-    }).toThrowError(
-      'NotAComponent(...): A valid React element (or null) must be returned. ' +
-        'You may have returned undefined, an array or some other invalid object.',
-    );
-  });
+  if (ReactDOMFeatureFlags.useFiber) {
+    it('should throw when stateless component returns undefined', () => {
+      function NotAComponent() {}
+      expect(function() {
+        ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
+      }).toThrowError(
+        'NotAComponent(...): Nothing was returned from render. ' +
+          'This usually means a return statement is missing. Or, to render nothing, return null.',
+      );
+    });
+  }
 
   it('should throw on string refs in pure functions', () => {
     function Child() {
diff --git a/src/renderers/__tests__/refs-test.js b/src/renderers/__tests__/refs-test.js
index 6f16f2fd34..3c920a3c50 100644
--- a/src/renderers/__tests__/refs-test.js
+++ b/src/renderers/__tests__/refs-test.js
@@ -394,11 +394,6 @@ describe('string refs between fiber and stack', () => {
 });
 
 describe('root level refs', () => {
-  beforeEach(() => {
-    var ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
-  });
-
   it('attaches and detaches root refs', () => {
     var ReactDOM = require('react-dom');
     var inst = null;
diff --git a/src/renderers/dom/ReactDOMNodeStreamRenderer.js b/src/renderers/dom/ReactDOMNodeStreamRenderer.js
index 7a54db6531..3fa43ead77 100644
--- a/src/renderers/dom/ReactDOMNodeStreamRenderer.js
+++ b/src/renderers/dom/ReactDOMNodeStreamRenderer.js
@@ -11,11 +11,7 @@
 
 'use strict';
 
-var invariant = require('fbjs/lib/invariant');
-var React = require('react');
 var ReactPartialRenderer = require('ReactPartialRenderer');
-var ReactFeatureFlags = require('ReactFeatureFlags');
-
 var Readable = require('stream').Readable;
 
 // This is a Readable Node.js stream which wraps the ReactDOMPartialRenderer.
@@ -41,13 +37,6 @@ class ReactMarkupReadableStream extends Readable {
  * See https://facebook.github.io/react/docs/react-dom-stream.html#rendertonodestream
  */
 function renderToNodeStream(element) {
-  const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-  if (disableNewFiberFeatures) {
-    invariant(
-      React.isValidElement(element),
-      'renderToNodeStream(): Invalid component element.',
-    );
-  }
   return new ReactMarkupReadableStream(element, false);
 }
 
@@ -57,13 +46,6 @@ function renderToNodeStream(element) {
  * See https://facebook.github.io/react/docs/react-dom-stream.html#rendertostaticnodestream
  */
 function renderToStaticNodeStream(element) {
-  const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-  if (disableNewFiberFeatures) {
-    invariant(
-      React.isValidElement(element),
-      'renderToStaticNodeStream(): Invalid component element.',
-    );
-  }
   return new ReactMarkupReadableStream(element, true);
 }
 
diff --git a/src/renderers/dom/ReactDOMStringRenderer.js b/src/renderers/dom/ReactDOMStringRenderer.js
index 4374e29721..94649df7c0 100644
--- a/src/renderers/dom/ReactDOMStringRenderer.js
+++ b/src/renderers/dom/ReactDOMStringRenderer.js
@@ -11,10 +11,7 @@
 
 'use strict';
 
-var invariant = require('fbjs/lib/invariant');
-var React = require('react');
 var ReactPartialRenderer = require('ReactPartialRenderer');
-var ReactFeatureFlags = require('ReactFeatureFlags');
 
 /**
  * Render a ReactElement to its initial HTML. This should only be used on the
@@ -22,13 +19,6 @@ var ReactFeatureFlags = require('ReactFeatureFlags');
  * See https://facebook.github.io/react/docs/react-dom-server.html#rendertostring
  */
 function renderToString(element) {
-  const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-  if (disableNewFiberFeatures) {
-    invariant(
-      React.isValidElement(element),
-      'renderToString(): Invalid component element.',
-    );
-  }
   var renderer = new ReactPartialRenderer(element, false);
   var markup = renderer.read(Infinity);
   return markup;
@@ -40,13 +30,6 @@ function renderToString(element) {
  * See https://facebook.github.io/react/docs/react-dom-server.html#rendertostaticmarkup
  */
 function renderToStaticMarkup(element) {
-  const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-  if (disableNewFiberFeatures) {
-    invariant(
-      React.isValidElement(element),
-      'renderToStaticMarkup(): Invalid component element.',
-    );
-  }
   var renderer = new ReactPartialRenderer(element, true);
   var markup = renderer.read(Infinity);
   return markup;
diff --git a/src/renderers/dom/__tests__/ReactDOMProduction-test.js b/src/renderers/dom/__tests__/ReactDOMProduction-test.js
index d4fab65a45..422ee37104 100644
--- a/src/renderers/dom/__tests__/ReactDOMProduction-test.js
+++ b/src/renderers/dom/__tests__/ReactDOMProduction-test.js
@@ -197,6 +197,7 @@ describe('ReactDOMProduction', () => {
   });
 
   it('should throw with an error code in production', () => {
+    const errorCode = ReactDOMFeatureFlags.useFiber ? 152 : 109;
     expect(function() {
       class Component extends React.Component {
         render() {
@@ -207,8 +208,8 @@ describe('ReactDOMProduction', () => {
       var container = document.createElement('div');
       ReactDOM.render(<Component />, container);
     }).toThrowError(
-      'Minified React error #109; visit ' +
-        'http://facebook.github.io/react/docs/error-decoder.html?invariant=109&args[]=Component' +
+      `Minified React error #${errorCode}; visit ` +
+        `http://facebook.github.io/react/docs/error-decoder.html?invariant=${errorCode}&args[]=Component` +
         ' for the full message or use the non-minified dev environment' +
         ' for full errors and additional helpful warnings.',
     );
diff --git a/src/renderers/dom/fiber/ReactDOMFiberEntry.js b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
index f340a2a6de..322a8262df 100644
--- a/src/renderers/dom/fiber/ReactDOMFiberEntry.js
+++ b/src/renderers/dom/fiber/ReactDOMFiberEntry.js
@@ -21,7 +21,6 @@ var ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
 var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
 var ReactControlledComponent = require('ReactControlledComponent');
 var ReactDOMComponentTree = require('ReactDOMComponentTree');
-var ReactFeatureFlags = require('ReactFeatureFlags');
 var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
 var ReactDOMFiberComponent = require('ReactDOMFiberComponent');
 var ReactDOMFrameScheduling = require('ReactDOMFrameScheduling');
@@ -31,7 +30,6 @@ var ReactInputSelection = require('ReactInputSelection');
 var ReactInstanceMap = require('ReactInstanceMap');
 var ReactPortal = require('ReactPortal');
 var ReactVersion = require('ReactVersion');
-var {isValidElement} = require('react');
 var {injectInternals} = require('ReactFiberDevToolsHook');
 var {
   ELEMENT_NODE,
@@ -659,38 +657,6 @@ var ReactDOMFiber = {
     container: DOMContainer,
     callback: ?Function,
   ) {
-    if (ReactFeatureFlags.disableNewFiberFeatures) {
-      // Top-level check occurs here instead of inside child reconciler
-      // because requirements vary between renderers. E.g. React Art
-      // allows arrays.
-      if (!isValidElement(element)) {
-        if (typeof element === 'string') {
-          invariant(
-            false,
-            'ReactDOM.render(): Invalid component element. Instead of ' +
-              "passing a string like 'div', pass " +
-              "React.createElement('div') or <div />.",
-          );
-        } else if (typeof element === 'function') {
-          invariant(
-            false,
-            'ReactDOM.render(): Invalid component element. Instead of ' +
-              'passing a class like Foo, pass React.createElement(Foo) ' +
-              'or <Foo />.',
-          );
-        } else if (element != null && typeof element.props !== 'undefined') {
-          // Check if it quacks like an element
-          invariant(
-            false,
-            'ReactDOM.render(): Invalid component element. This may be ' +
-              'caused by unintentionally loading two independent copies ' +
-              'of React.',
-          );
-        } else {
-          invariant(false, 'ReactDOM.render(): Invalid component element.');
-        }
-      }
-    }
     return renderSubtreeIntoContainer(
       null,
       element,
diff --git a/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js b/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
index 2910041d9b..aabaeae11f 100644
--- a/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
+++ b/src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
@@ -23,17 +23,9 @@ describe('ReactDOMFiber', () => {
   }
 
   var container;
-  var ReactFeatureFlags;
 
   beforeEach(() => {
     container = document.createElement('div');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
-  });
-
-  afterEach(() => {
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = true;
   });
 
   it('should render strings as children', () => {
@@ -1103,68 +1095,3 @@ describe('ReactDOMFiber', () => {
     });
   }
 });
-
-// disableNewFiberFeatures currently defaults to true in test
-describe('disableNewFiberFeatures', () => {
-  var container;
-  var ReactFeatureFlags;
-
-  beforeEach(() => {
-    container = document.createElement('div');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = true;
-  });
-
-  afterEach(() => {
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
-  });
-
-  it('throws if non-element passed to top-level render', () => {
-    const message = 'render(): Invalid component element.';
-    expect(() => ReactDOM.render(null, container)).toThrow(message, container);
-    expect(() => ReactDOM.render(undefined, container)).toThrow(
-      message,
-      container,
-    );
-    expect(() => ReactDOM.render(false, container)).toThrow(message, container);
-    expect(() => ReactDOM.render('Hi', container)).toThrow(message, container);
-    expect(() => ReactDOM.render(999, container)).toThrow(message, container);
-    expect(() => ReactDOM.render([<div key="a" />], container)).toThrow(
-      message,
-      container,
-    );
-  });
-
-  it('throws if something other than false, null, or an element is returned from render', () => {
-    function Render(props) {
-      return props.children;
-    }
-
-    expect(() => ReactDOM.render(<Render>Hi</Render>, container)).toThrow(
-      /You may have returned undefined/,
-    );
-    expect(() => ReactDOM.render(<Render>{999}</Render>, container)).toThrow(
-      /You may have returned undefined/,
-    );
-    expect(() =>
-      ReactDOM.render(<Render>[<div key="a" />]</Render>, container),
-    ).toThrow(/You may have returned undefined/);
-  });
-
-  it('treats mocked render functions as if they return null', () => {
-    class Mocked extends React.Component {}
-    Mocked.prototype.render = jest.fn();
-    ReactDOM.render(<Mocked />, container);
-    expect(container.textContent).toEqual('');
-  });
-
-  it('throws if the React package cannot be loaded', () => {
-    jest.resetModules();
-    jest.mock('react', () => undefined);
-    expect(() => require('react-dom')).toThrow(
-      'ReactDOM was loaded before React.',
-    );
-    jest.resetModules();
-  });
-});
diff --git a/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js b/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
index 0d42561170..4f8b327ad4 100644
--- a/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactDOMServerIntegration-test.js
@@ -323,7 +323,6 @@ function resetModules() {
   // TODO: can we express this test with only public API?
   ExecutionEnvironment = require('ExecutionEnvironment');
 
-  require('ReactFeatureFlags').disableNewFiberFeatures = false;
   PropTypes = require('prop-types');
   React = require('react');
   ReactDOM = require('react-dom');
@@ -334,7 +333,6 @@ function resetModules() {
   // Resetting is important because we want to avoid any shared state
   // influencing the tests.
   jest.resetModuleRegistry();
-  require('ReactFeatureFlags').disableNewFiberFeatures = false;
   ReactDOMServer = require('react-dom/server');
 }
 
diff --git a/src/renderers/dom/shared/__tests__/ReactMount-test.js b/src/renderers/dom/shared/__tests__/ReactMount-test.js
index 458cbd09ff..07b288de27 100644
--- a/src/renderers/dom/shared/__tests__/ReactMount-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactMount-test.js
@@ -68,29 +68,24 @@ describe('ReactMount', () => {
     });
   });
 
-  it('throws when given a string', () => {
-    expect(function() {
-      ReactTestUtils.renderIntoDocument('div');
-    }).toThrowError(
-      'ReactDOM.render(): Invalid component element. Instead of passing a ' +
-        "string like 'div', pass React.createElement('div') or <div />.",
-    );
-  });
-
-  it('throws when given a factory', () => {
-    class Component extends React.Component {
-      render() {
-        return <div />;
+  if (ReactDOMFeatureFlags.useFiber) {
+    it('warns when given a factory', () => {
+      spyOn(console, 'error');
+      class Component extends React.Component {
+        render() {
+          return <div />;
+        }
       }
-    }
 
-    expect(function() {
       ReactTestUtils.renderIntoDocument(Component);
-    }).toThrowError(
-      'ReactDOM.render(): Invalid component element. Instead of passing a ' +
-        'class like Foo, pass React.createElement(Foo) or <Foo />.',
-    );
-  });
+      expectDev(console.error.calls.count()).toBe(1);
+      expectDev(console.error.calls.argsFor(0)[0]).toContain(
+        'Functions are not valid as a React child. ' +
+          'This may happen if you return a Component instead of <Component /> from render. ' +
+          'Or maybe you meant to call this function rather than return it.',
+      );
+    });
+  }
 
   it('should render different components in same root', () => {
     var container = document.createElement('container');
@@ -401,9 +396,6 @@ describe('ReactMount', () => {
       let mountPoint;
 
       beforeEach(() => {
-        const ReactFeatureFlags = require('ReactFeatureFlags');
-        ReactFeatureFlags.disableNewFiberFeatures = false;
-
         containerDiv = document.createElement('div');
         containerDiv.innerHTML = 'A<!-- react-mount-point-unstable -->B';
         mountPoint = containerDiv.childNodes[1];
diff --git a/src/renderers/dom/shared/__tests__/ReactServerRendering-test.js b/src/renderers/dom/shared/__tests__/ReactServerRendering-test.js
index cb3046b5e4..1e273f8ecb 100644
--- a/src/renderers/dom/shared/__tests__/ReactServerRendering-test.js
+++ b/src/renderers/dom/shared/__tests__/ReactServerRendering-test.js
@@ -19,7 +19,6 @@ var ReactMarkupChecksum;
 var ReactReconcileTransaction;
 var ReactTestUtils;
 var PropTypes;
-var ReactFeatureFlags;
 
 var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
 
@@ -36,9 +35,6 @@ describe('ReactDOMServer', () => {
     ReactReconcileTransaction = require('ReactReconcileTransaction');
     PropTypes = require('prop-types');
 
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
-
     ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
     ExecutionEnvironment.canUseDOM = false;
     ReactDOMServer = require('react-dom/server');
diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index 9a4157b13e..655e2a1e85 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -27,7 +27,6 @@ var ReactTypeOfWork = require('ReactTypeOfWork');
 
 var emptyObject = require('fbjs/lib/emptyObject');
 var invariant = require('fbjs/lib/invariant');
-var ReactFeatureFlags = require('ReactFeatureFlags');
 
 if (__DEV__) {
   var {getCurrentFiberStackAddendum} = require('ReactDebugCurrentFiber');
@@ -576,8 +575,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     }
 
     if (__DEV__) {
-      const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-      if (!disableNewFiberFeatures && typeof newChild === 'function') {
+      if (typeof newChild === 'function') {
         warnOnFunctionType();
       }
     }
@@ -656,8 +654,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     }
 
     if (__DEV__) {
-      const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-      if (!disableNewFiberFeatures && typeof newChild === 'function') {
+      if (typeof newChild === 'function') {
         warnOnFunctionType();
       }
     }
@@ -722,8 +719,7 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     }
 
     if (__DEV__) {
-      const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-      if (!disableNewFiberFeatures && typeof newChild === 'function') {
+      if (typeof newChild === 'function') {
         warnOnFunctionType();
       }
     }
@@ -1301,118 +1297,51 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     // not as a fragment. Nested arrays on the other hand will be treated as
     // fragment nodes. Recursion happens at the normal flow.
 
-    const disableNewFiberFeatures = ReactFeatureFlags.disableNewFiberFeatures;
-
     // Handle object types
     const isObject = typeof newChild === 'object' && newChild !== null;
     if (isObject) {
       // Support only the subset of return types that Stack supports. Treat
       // everything else as empty, but log a warning.
-      if (disableNewFiberFeatures) {
-        switch (newChild.$$typeof) {
-          case REACT_ELEMENT_TYPE:
-            return placeSingleChild(
-              reconcileSingleElement(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-
-          case REACT_PORTAL_TYPE:
-            return placeSingleChild(
-              reconcileSinglePortal(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-        }
-      } else {
-        switch (newChild.$$typeof) {
-          case REACT_ELEMENT_TYPE:
-            return placeSingleChild(
-              reconcileSingleElement(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-
-          case REACT_COROUTINE_TYPE:
-            return placeSingleChild(
-              reconcileSingleCoroutine(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-
-          case REACT_YIELD_TYPE:
-            return placeSingleChild(
-              reconcileSingleYield(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-
-          case REACT_PORTAL_TYPE:
-            return placeSingleChild(
-              reconcileSinglePortal(
-                returnFiber,
-                currentFirstChild,
-                newChild,
-                priority,
-              ),
-            );
-        }
-      }
-    }
-
-    if (disableNewFiberFeatures) {
-      // The new child is not an element. If it's not null or false,
-      // and the return fiber is a composite component, throw an error.
-      switch (returnFiber.tag) {
-        case ClassComponent: {
-          if (__DEV__) {
-            const instance = returnFiber.stateNode;
-            if (
-              instance.render._isMockFunction &&
-              typeof newChild === 'undefined'
-            ) {
-              // We allow auto-mocks to proceed as if they're
-              // returning null.
-              break;
-            }
-          }
-          const Component = returnFiber.type;
-          invariant(
-            newChild === null || newChild === false,
-            '%s.render(): A valid React element (or null) must be returned. ' +
-              'You may have returned undefined, an array or some other ' +
-              'invalid object.',
-            Component.displayName || Component.name || 'Component',
+      switch (newChild.$$typeof) {
+        case REACT_ELEMENT_TYPE:
+          return placeSingleChild(
+            reconcileSingleElement(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority,
+            ),
           );
-          break;
-        }
-        case FunctionalComponent: {
-          // Composites accept elements, portals, null, or false
-          const Component = returnFiber.type;
-          invariant(
-            newChild === null || newChild === false,
-            '%s(...): A valid React element (or null) must be returned. ' +
-              'You may have returned undefined, an array or some other ' +
-              'invalid object.',
-            Component.displayName || Component.name || 'Component',
+
+        case REACT_COROUTINE_TYPE:
+          return placeSingleChild(
+            reconcileSingleCoroutine(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority,
+            ),
+          );
+
+        case REACT_YIELD_TYPE:
+          return placeSingleChild(
+            reconcileSingleYield(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority,
+            ),
+          );
+
+        case REACT_PORTAL_TYPE:
+          return placeSingleChild(
+            reconcileSinglePortal(
+              returnFiber,
+              currentFirstChild,
+              newChild,
+              priority,
+            ),
           );
-          break;
-        }
       }
     }
 
@@ -1450,11 +1379,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
     }
 
     if (__DEV__) {
-      if (!disableNewFiberFeatures && typeof newChild === 'function') {
+      if (typeof newChild === 'function') {
         warnOnFunctionType();
       }
     }
-    if (!disableNewFiberFeatures && typeof newChild === 'undefined') {
+    if (typeof newChild === 'undefined') {
       // If the new child is undefined, and the return fiber is a composite
       // component, throw an error. If Fiber return types are disabled,
       // we already threw above.
diff --git a/src/renderers/shared/fiber/__tests__/ReactCoroutine-test.js b/src/renderers/shared/fiber/__tests__/ReactCoroutine-test.js
index a331ffcc44..a90433a9e9 100644
--- a/src/renderers/shared/fiber/__tests__/ReactCoroutine-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactCoroutine-test.js
@@ -14,7 +14,6 @@
 var React;
 var ReactNoop;
 var ReactCoroutine;
-var ReactFeatureFlags;
 
 describe('ReactCoroutine', () => {
   beforeEach(() => {
@@ -23,8 +22,6 @@ describe('ReactCoroutine', () => {
     ReactNoop = require('react-noop-renderer');
     // TODO: can we express this test with only public API?
     ReactCoroutine = require('ReactCoroutine');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function div(...children) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
index e1a8dfc54c..ccba6727f5 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncremental-test.js
@@ -13,7 +13,6 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 var PropTypes;
 
 describe('ReactIncremental', () => {
@@ -22,9 +21,6 @@ describe('ReactIncremental', () => {
     React = require('react');
     ReactNoop = require('react-noop-renderer');
     PropTypes = require('prop-types');
-
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   it('should render a simple component', () => {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
index 433d3d6bc3..cc4882d626 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
@@ -14,7 +14,6 @@
 var PropTypes;
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalErrorHandling', () => {
   beforeEach(() => {
@@ -22,8 +21,6 @@ describe('ReactIncrementalErrorHandling', () => {
     PropTypes = require('prop-types');
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function div(...children) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalPerf-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalPerf-test.js
index c04b4782e7..b8908ea4c6 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalPerf-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalPerf-test.js
@@ -14,7 +14,6 @@
 describe('ReactDebugFiberPerf', () => {
   let React;
   let ReactCoroutine;
-  let ReactFeatureFlags;
   let ReactNoop;
   let ReactPortal;
   let PropTypes;
@@ -121,8 +120,6 @@ describe('ReactDebugFiberPerf', () => {
     // TODO: can we express this test with only public API?
     ReactCoroutine = require('ReactCoroutine');
     ReactPortal = require('ReactPortal');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
     PropTypes = require('prop-types');
   });
 
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalReflection-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalReflection-test.js
index 3181061fa8..f505d132de 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalReflection-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalReflection-test.js
@@ -13,15 +13,12 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalReflection', () => {
   beforeEach(() => {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   it('handles isMounted even when the initial render is deferred', () => {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
index d73ebe4133..3ed31d9456 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalScheduling-test.js
@@ -13,15 +13,12 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalScheduling', () => {
   beforeEach(() => {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function span(prop) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
index c15489376c..fd2266329d 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalSideEffects-test.js
@@ -13,15 +13,12 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalSideEffects', () => {
   beforeEach(() => {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function normalizeCodeLocInfo(str) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalTriangle-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalTriangle-test.js
index ddc2337ed2..995a32b9f1 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalTriangle-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalTriangle-test.js
@@ -13,16 +13,12 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalTriangle', () => {
   beforeEach(() => {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('ReactNoopEntry');
-
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function span(prop) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactIncrementalUpdates-test.js b/src/renderers/shared/fiber/__tests__/ReactIncrementalUpdates-test.js
index c4f980c517..7879162ad5 100644
--- a/src/renderers/shared/fiber/__tests__/ReactIncrementalUpdates-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactIncrementalUpdates-test.js
@@ -13,15 +13,12 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 describe('ReactIncrementalUpdates', () => {
   beforeEach(() => {
     jest.resetModuleRegistry();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   function span(prop) {
diff --git a/src/renderers/shared/fiber/__tests__/ReactTopLevelFragment-test.js b/src/renderers/shared/fiber/__tests__/ReactTopLevelFragment-test.js
index 7f1fb9f3cc..f09c5fafeb 100644
--- a/src/renderers/shared/fiber/__tests__/ReactTopLevelFragment-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactTopLevelFragment-test.js
@@ -13,7 +13,6 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 // This is a new feature in Fiber so I put it in its own test file. It could
 // probably move to one of the other test files once it is official.
@@ -22,8 +21,6 @@ describe('ReactTopLevelFragment', function() {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   it('should render a simple fragment at the top of a component', function() {
diff --git a/src/renderers/shared/fiber/__tests__/ReactTopLevelText-test.js b/src/renderers/shared/fiber/__tests__/ReactTopLevelText-test.js
index 24ab990946..1e2e8eb5a1 100644
--- a/src/renderers/shared/fiber/__tests__/ReactTopLevelText-test.js
+++ b/src/renderers/shared/fiber/__tests__/ReactTopLevelText-test.js
@@ -13,7 +13,6 @@
 
 var React;
 var ReactNoop;
-var ReactFeatureFlags;
 
 // This is a new feature in Fiber so I put it in its own test file. It could
 // probably move to one of the other test files once it is official.
@@ -22,8 +21,6 @@ describe('ReactTopLevelText', () => {
     jest.resetModules();
     React = require('react');
     ReactNoop = require('react-noop-renderer');
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
   });
 
   it('should render a component returning strings directly from render', () => {
diff --git a/src/renderers/shared/utils/ReactFeatureFlags.js b/src/renderers/shared/utils/ReactFeatureFlags.js
index e0c72dd380..7e19b31dee 100644
--- a/src/renderers/shared/utils/ReactFeatureFlags.js
+++ b/src/renderers/shared/utils/ReactFeatureFlags.js
@@ -13,7 +13,6 @@
 'use strict';
 
 var ReactFeatureFlags = {
-  disableNewFiberFeatures: false,
   enableAsyncSubtreeAPI: true,
 };
 
diff --git a/src/renderers/testing/__tests__/ReactTestRenderer-test.js b/src/renderers/testing/__tests__/ReactTestRenderer-test.js
index 163bdabd6b..566a0788a5 100644
--- a/src/renderers/testing/__tests__/ReactTestRenderer-test.js
+++ b/src/renderers/testing/__tests__/ReactTestRenderer-test.js
@@ -14,7 +14,6 @@
 var React = require('react');
 var ReactTestRenderer = require('react-test-renderer');
 var prettyFormat = require('pretty-format');
-var ReactFeatureFlags;
 
 // Kind of hacky, but we nullify all the instances to test the tree structure
 // with jasmine's deep equality function, and test the instances separate. We
@@ -40,11 +39,6 @@ function cleanNode(node) {
 }
 
 describe('ReactTestRenderer', () => {
-  beforeEach(() => {
-    ReactFeatureFlags = require('ReactFeatureFlags');
-    ReactFeatureFlags.disableNewFiberFeatures = false;
-  });
-
   function normalizeCodeLocInfo(str) {
     return str && str.replace(/\(at .+?:\d+\)/g, '(at **)');
   }