Fix memory leak in ReactChildrenMutationWarningHook for SSR (#7410)

* corrected ReactChildrenMutationWarningHook's name

* changed `onComponentHasMounted` to `onMountComponent`

and get element from `ReactComponentTreeHook` instead of keeping an internal store

src/renderers/dom/server/__tests__/ReactServerRendering-test.js

@@ -538,4 +538,38 @@ describe('ReactServerRendering', function() {
     var markup = ReactServerRendering.renderToStaticMarkup(<Baz />);
     expect(markup).toBe('<div></div>');
   });
+
+  it('warns when children are mutated before render', function() {
+    function normalizeCodeLocInfo(str) {
+      return str.replace(/\(at .+?:\d+\)/g, '(at **)');
+    }
+
+    spyOn(console, 'error');
+    var children = [<span key={0} />, <span key={1} />, <span key={2} />];
+    var element = <div>{children}</div>;
+    children[1] = <p key={1} />; // Mutation is illegal
+    ReactServerRendering.renderToString(element);
+    expect(console.error.calls.count()).toBe(1);
+    expect(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
+      'Warning: Component\'s children should not be mutated.\n    in div (at **)'
+    );
+  });
+
+  it('should warn when children are mutated', function() {
+    function normalizeCodeLocInfo(str) {
+      return str.replace(/\(at .+?:\d+\)/g, '(at **)');
+    }
+
+    spyOn(console, 'error');
+    var children = [<span key={0} />, <span key={1} />, <span key={2} />];
+    function Wrapper(props) {
+      props.children[1] = <p key={1} />; // Mutation is illegal
+      return <div>{props.children}</div>;
+    }
+    ReactServerRendering.renderToString(<Wrapper>{children}</Wrapper>);
+    expect(console.error.calls.count()).toBe(1);
+    expect(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
+      'Warning: Component\'s children should not be mutated.\n    in Wrapper (at **)'
+    );
+  });
 });

src/renderers/shared/hooks/ReactChildrenMutationWarningHook.js

@@ -15,8 +15,6 @@ var ReactComponentTreeHook = require('ReactComponentTreeHook');
 
 var warning = require('warning');
 
-var elements = {};
-
 function handleElement(debugID, element) {
   if (element == null) {
     return;
@@ -46,21 +44,13 @@ function handleElement(debugID, element) {
   );
 }
 
-var ReactDOMUnknownPropertyHook = {
-  onBeforeMountComponent(debugID, element) {
-    elements[debugID] = element;
-  },
-  onBeforeUpdateComponent(debugID, element) {
-    elements[debugID] = element;
-  },
-  onComponentHasMounted(debugID) {
-    handleElement(debugID, elements[debugID]);
-    delete elements[debugID];
+var ReactChildrenMutationWarningHook = {
+  onMountComponent(debugID) {
+    handleElement(debugID, ReactComponentTreeHook.getElement(debugID));
   },
   onComponentHasUpdated(debugID) {
-    handleElement(debugID, elements[debugID]);
-    delete elements[debugID];
+    handleElement(debugID, ReactComponentTreeHook.getElement(debugID));
   },
 };
 
-module.exports = ReactDOMUnknownPropertyHook;
+module.exports = ReactChildrenMutationWarningHook;