instance.unstable_asyncUpdates = true creates an async subtree

Same behavior as unstable_asyncRender. Can be used to wrap a specific
subtree, rather than an entire root.

src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js

@@ -70,6 +70,52 @@ describe('ReactDOMFiberAsync', () => {
         jest.runAllTimers();
         expect(container.textContent).toEqual('1');
       });
+
+      it('unstable_asyncUpdates creates an async subtree', () => {
+        let instance;
+        class Component extends React.Component {
+          state = {step: 0};
+          unstable_asyncUpdates = true;
+          render() {
+            instance = this;
+            return <div>{this.state.step}</div>;
+          }
+        }
+
+        ReactDOM.render(<Component />, container);
+        jest.runAllTimers();
+
+        instance.setState({step: 1});
+        expect(container.textContent).toEqual('0');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('1');
+      });
+
+      it('updates inside an async subtree are async by default', () => {
+        class Component extends React.Component {
+          unstable_asyncUpdates = true;
+          render() {
+            return <Child />;
+          }
+        }
+
+        let instance;
+        class Child extends React.Component {
+          state = {step: 0};
+          render() {
+            instance = this;
+            return <div>{this.state.step}</div>;
+          }
+        }
+
+        ReactDOM.render(<Component />, container);
+        jest.runAllTimers();
+
+        instance.setState({step: 1});
+        expect(container.textContent).toEqual('0');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('1');
+      })
     });
   }
 });

src/renderers/shared/fiber/ReactFiberClassComponent.js

@@ -305,6 +305,10 @@ module.exports = function(
     instance.refs = emptyObject;
     instance.context = getMaskedContext(workInProgress, unmaskedContext);
 
+    if (instance.unstable_asyncUpdates === true) {
+      workInProgress.contextTag |= AsyncUpdates;
+    }
+
     if (typeof instance.componentWillMount === 'function') {
       if (__DEV__) {
         startPhaseTimer(workInProgress, 'componentWillMount');