Add 'flushSync' to Draft.js for *only* GK folks

Summary:
We did this in https://our.intern.facebook.com/intern/diff/D6727557/ and then it
was reverted due to bugs like
https://our.intern.facebook.com/intern/tasks/?t=25244286
... But we still want to do this! Let's enable under a GK so we can dogfood and
possibly identify repro steps.

Context from original version of this diff -
We have seen various issues with Draft and other controlled text inputs when
React is in "async mode", such that updates are rendered asynchronously.
For more context and repro steps, see #1577

I also added the logic to the plain text editor test page for manual testing, until we write unit tests for this.

Reviewed By: acdlite

Differential Revision: D6802653

fbshipit-source-id: 9f6952e9cb77cf79f9650d167075b1c337e3d35f

Author: flarnie

src/component/base/DraftEditor.react.js

@@ -26,6 +26,7 @@ const DraftEditorContents = require('DraftEditorContents.react');
 const DraftEditorDragHandler = require('DraftEditorDragHandler');
 const DraftEditorEditHandler = require('DraftEditorEditHandler');
 const DraftEditorPlaceholder = require('DraftEditorPlaceholder.react');
+const DraftFeatureFlags = require('DraftFeatureFlags');
 const EditorState = require('EditorState');
 const React = require('React');
 const ReactDOM = require('ReactDOM');
@@ -171,10 +172,15 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
    * editor mode, if any has been specified.
    */
   _buildHandler(eventName: string): Function {
+    const flushSyncSupported = !!ReactDOM.flushSync;
     return e => {
       if (!this.props.readOnly) {
         const method = this._handler && this._handler[eventName];
-        method && method(this, e);
+        if (flushSyncSupported && DraftFeatureFlags.draft_js_flush_sync) {
+          method && ReactDOM.flushSync(() => method(this, e));
+        } else {
+          method && method(this, e);
+        }
       }
     };
   }

src/component/utils/DraftFeatureFlags-core.js

@@ -18,6 +18,7 @@ var DraftFeatureFlags = {
   draft_segmented_entities_behavior: false,
   draft_handlebeforeinput_composed_text: false,
   draft_tree_data_support: false,
+  draft_js_flush_sync: false,
 };
 
 module.exports = DraftFeatureFlags;