Add text directionality override prop to DraftEditor

Credit for this change goes to @mmmoussa

 - Added an optional textDirectionality prop of type
   DraftTextDirectionality to the DraftEditor component which allows
   overriding the directionality of all blocks in the editor.
 - Fixed lint errors in the modified Draft.js files.

Note that we will be updating the DraftTextDirectionality type
definition to move it to the `fbjs` shared utils, possibly along with
related text directionality utilities.

docs/APIReference-Editor.md

@@ -57,6 +57,19 @@ to fit it within your UI design.
 If this value is not set, text alignment will be based on the characters within
 the editor, on a per-block basis.
 
+#### textDirectionality
+```
+textDirectionality?: DraftTextDirectionality
+```
+Optionally set the overriding text directionality for this editor. The values
+include 'RTL' for right-to-left text, like Hebrew or Arabic, and 'LTR' for
+left-to-right text, like English or Spanish. This directionality will apply to
+the entire contents, regardless of default text direction for input text.
+
+If this value is not set, text directionality will be based on the characters
+within the editor, on a per-block basis.
+
+
 #### blockRendererFn
 ```
 blockRendererFn?: (block: ContentBlock) => ?Object

src/component/base/DraftEditor.react.js

@@ -283,6 +283,7 @@ class DraftEditor extends React.Component {
               editorKey={this._editorKey}
               editorState={this.props.editorState}
               key={'contents' + this.state.contentsKey}
+              textDirectionality={this.props.textDirectionality}
             />
           </div>
         </div>
@@ -355,8 +356,8 @@ class DraftEditor extends React.Component {
       this.update(
         EditorState.forceSelection(
           editorState,
-          editorState.getSelection()
-        )
+          editorState.getSelection(),
+        ),
       );
     }
   }

src/component/base/DraftEditorProps.js

@@ -17,6 +17,7 @@ import type {DraftBlockRenderMap} from 'DraftBlockRenderMap';
 import type {DraftDragType} from 'DraftDragType';
 import type {DraftEditorCommand} from 'DraftEditorCommand';
 import type {DraftTextAlignment} from 'DraftTextAlignment';
+import type {DraftTextDirectionality} from 'DraftTextDirectionality';
 import type {DraftInlineStyle} from 'DraftInlineStyle';
 import type {DraftHandleValue} from 'DraftHandleValue';
 import type EditorState from 'EditorState';
@@ -42,6 +43,10 @@ export type DraftEditorProps = {
   // regardless of input characters.
   textAlignment?: DraftTextAlignment,
 
+  // Specify whether text directionality should be forced in a direction
+  // regardless of input characters.
+  textDirectionality?: DraftTextDirectionality,
+
   // For a given `ContentBlock` object, return an object that specifies
   // a custom block component and/or props. If no object is returned,
   // the default `TextEditorBlock` is used.

src/component/base/DraftTextDirectionality.js

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule DraftTextDirectionality
+ * @flow
+ */
+
+'use strict';
+
+// TODO: (t16333390) import this type from 'fbjs'
+export type DraftTextDirectionality = 'LTR' | 'RTL' | 'NEUTRAL';

src/component/contents/DraftEditorContents.react.js

@@ -29,6 +29,7 @@ type Props = {
   blockRendererFn: Function,
   blockStyleFn: (block: ContentBlock) => string,
   editorState: EditorState,
+  textDirectionality?: BidiDirection,
 };
 
 /**
@@ -123,7 +124,10 @@ class DraftEditorContents extends React.Component {
         customEditable = customRenderer.editable;
       }
 
-      const direction = directionMap.get(key);
+      const {textDirectionality} = this.props;
+      const direction = textDirectionality
+        ? textDirectionality
+        : directionMap.get(key);
       const offsetKey = DraftOffsetKey.encode(key, 0, 0);
       const componentProps = {
         contentState: content,
@@ -161,7 +165,7 @@ class DraftEditorContents extends React.Component {
         );
         className = joinClasses(
           className,
-          getListItemClasses(blockType, depth, shouldResetCount, direction)
+          getListItemClasses(blockType, depth, shouldResetCount, direction),
         );
       }
 
@@ -221,7 +225,7 @@ class DraftEditorContents extends React.Component {
             key: info.key + '-wrap',
             'data-offset-key': info.offsetKey,
           },
-          blocks
+          blocks,
         );
         outputBlocks.push(wrapperElement);
       } else {
@@ -244,7 +248,7 @@ function getListItemClasses(
   type: string,
   depth: number,
   shouldResetCount: boolean,
-  direction: BidiDirection
+  direction: BidiDirection,
 ): string {
   return cx({
     'public/DraftStyleDefault/unorderedListItem':