Remove legacy React element warning

scripts/fiber/tests-passing.txt

@@ -41,7 +41,6 @@ src/addons/__tests__/ReactComponentWithPureRenderMixin-test.js
 src/addons/__tests__/ReactFragment-test.js
 * should throw if a plain object is used as a child
 * should throw if a plain object even if it is in an owner
-* should throw if a plain object looks like an old element
 * warns for numeric keys on objects as children
 * should warn if passing null to createFragment
 * should warn if passing an array to createFragment

src/addons/__tests__/ReactFragment-test.js

@@ -59,17 +59,6 @@ describe('ReactFragment', () => {
     );
   });
 
-  it('should throw if a plain object looks like an old element', () => {
-    var oldEl = {_isReactElement: true, type: 'span', props: {}};
-    var container = document.createElement('div');
-    expect(() => ReactDOM.render(<div>{oldEl}</div>, container)).toThrowError(
-      'Objects are not valid as a React child (found: object with keys ' +
-      '{_isReactElement, type, props}). It looks like you\'re using an ' +
-      'element created by a different version of React. Make sure to use ' +
-      'only one copy of React.'
-    );
-  });
-
   it('warns for numeric keys on objects as children', () => {
     spyOn(console, 'error');
 

src/renderers/shared/fiber/ReactChildFiber.js

@@ -118,11 +118,6 @@ function throwOnInvalidObjectType(returnFiber : Fiber, newChild : Object) {
         ' If you meant to render a collection of children, use an array ' +
         'instead or wrap the object using createFragment(object) from the ' +
         'React add-ons.';
-      if (newChild._isReactElement) {
-        addendum =
-          ' It looks like you\'re using an element created by a different ' +
-          'version of React. Make sure to use only one copy of React.';
-      }
       const owner = ReactCurrentOwner.owner || returnFiber._debugOwner;
       if (owner && typeof owner.tag === 'number') {
         const name = getComponentName((owner : any));

src/shared/utils/traverseAllChildren.js

@@ -167,11 +167,6 @@ function traverseAllChildrenImpl(
           ' If you meant to render a collection of children, use an array ' +
           'instead or wrap the object using createFragment(object) from the ' +
           'React add-ons.';
-        if (children._isReactElement) {
-          addendum =
-            ' It looks like you\'re using an element created by a different ' +
-            'version of React. Make sure to use only one copy of React.';
-        }
         if (ReactCurrentOwner.current) {
           var name = ReactCurrentOwner.current.getName();
           if (name) {