Fix return value of ReactDOMFiber.unmountComponentAtNode (#9619)

scripts/fiber/tests-passing.txt

@@ -1240,6 +1240,8 @@ src/renderers/dom/shared/__tests__/ReactEventListener-test.js
 
 src/renderers/dom/shared/__tests__/ReactMount-test.js
 * throws when given a non-node
+* returns false on non-React containers
+* returns true on React containers
 * throws when given a string
 * throws when given a factory
 * should render different components in same root

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -518,11 +518,16 @@ var ReactDOM = {
       }
 
       // Unmount should not be batched.
-      return DOMRenderer.unbatchedUpdates(() => {
-        return renderSubtreeIntoContainer(null, null, container, () => {
+      DOMRenderer.unbatchedUpdates(() => {
+        renderSubtreeIntoContainer(null, null, container, () => {
           container._reactRootContainer = null;
         });
       });
+      // If you call unmountComponentAtNode twice in quick succession, you'll
+      // get `true` twice. That's probably fine?
+      return true;
+    } else {
+      return false;
     }
   },
 

src/renderers/dom/shared/__tests__/ReactMount-test.js

@@ -46,6 +46,21 @@ describe('ReactMount', () => {
         'unmountComponentAtNode(...): Target container is not a DOM element.',
       );
     });
+
+    it('returns false on non-React containers', () => {
+      var d = document.createElement('div');
+      d.innerHTML = '<b>hellooo</b>';
+      expect(ReactDOM.unmountComponentAtNode(d)).toBe(false);
+      expect(d.textContent).toBe('hellooo');
+    });
+
+    it('returns true on React containers', () => {
+      var d = document.createElement('div');
+      ReactDOM.render(<b>hellooo</b>, d);
+      expect(d.textContent).toBe('hellooo');
+      expect(ReactDOM.unmountComponentAtNode(d)).toBe(true);
+      expect(d.textContent).toBe('');
+    });
   });
 
   it('throws when given a string', () => {