Adds warning for older unstable methods

Author: ManasJayanth

packages/react-dom/src/__tests__/ReactDOMFiber-test.js

@@ -222,6 +222,7 @@ describe('ReactDOMFiber', () => {
 
   // TODO: remove in React 17
   it('should support unstable_createPortal alias', () => {
+    spyOnDev(console, 'warn');
     var portalContainer = document.createElement('div');
 
     ReactDOM.render(
@@ -233,6 +234,14 @@ describe('ReactDOMFiber', () => {
     expect(portalContainer.innerHTML).toBe('<div>portal</div>');
     expect(container.innerHTML).toBe('<div></div>');
 
+    if (__DEV__) {
+      expect(console.warn.calls.count()).toBe(1);
+      expect(console.warn.calls.argsFor(0)[0]).toContain(
+        'unstable_createPortal is merely an alias to createPortal ' +
+          'and will not work in React 17+. Please update your code.',
+      );
+    }
+
     ReactDOM.unmountComponentAtNode(container);
     expect(portalContainer.innerHTML).toBe('');
     expect(container.innerHTML).toBe('');

packages/react-dom/src/client/ReactDOM.js

@@ -1263,7 +1263,14 @@ const ReactDOM: Object = {
 
   // Temporary alias since we already shipped React 16 RC with it.
   // TODO: remove in React 17.
-  unstable_createPortal: createPortal,
+  unstable_createPortal(...args) {
+    lowPriorityWarning(
+      false,
+      'unstable_createPortal is merely an alias to createPortal ' +
+        'and will not work in React 17+. Please update your code.',
+    );
+    return createPortal(...args);
+  },
 
   unstable_batchedUpdates: ReactGenericBatching.batchedUpdates,