Silence Fiber warning when the feature flag is on

When the feature flag is on, we should silence the warning since
we're explicitly testing it. This is needed when running unit
tests with the flag on.

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -15,6 +15,7 @@
 import type { HostChildren } from 'ReactFiberReconciler';
 
 var ReactFiberReconciler = require('ReactFiberReconciler');
+var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
 
 var warning = require('warning');
 
@@ -98,8 +99,9 @@ var DOMRenderer = ReactFiberReconciler({
 var warned = false;
 
 function warnAboutUnstableUse() {
+  // Ignore this warning is the feature flag is turned on. E.g. for tests.
   warning(
-    warned,
+    warned || ReactDOMFeatureFlags.useFiber,
     'You are using React DOM Fiber which is an experimental renderer. ' +
     'It is likely to have bugs, breaking changes and is unsupported.'
   );