Actually for real fix the flow type of decorator props

Summary:
The 'entityKey' could always have been null/undefined, and it's time that the
flow type reflects that. Our code should handle the case, though unexpected,
when it could be undefined.

A previous diff[1] was submitted but somehow the changes were not all commited.
This finishes the job, updating decorator components in our product code and the
type definition in Draft.js.

[1]: https://our.intern.facebook.com/intern/diff/D7659273/

Reviewed By: sophiebits

Differential Revision: D7710224

fbshipit-source-id: 7c673a565f79e45bced91db5b406974f9813327d

src/component/contents/DraftEditorBlock.react.js

@@ -207,7 +207,6 @@ class DraftEditorBlock extends React.Component<Props> {
           start,
           end,
           blockKey,
-          // $FlowFixMe
           entityKey,
           offsetKey: decoratorOffsetKey,
         };

src/model/decorators/DraftDecorator.js

@@ -59,7 +59,16 @@ export type DraftDecoratorComponentProps = {
   decoratedText: string,
   dir: ?HTMLDir,
   end: number,
-  entityKey: string,
+
+  // Many folks mistakenly assume that there will always be an 'entityKey'
+  // passed to a DecoratorComponent.
+  // To find the `entityKey`, Draft calls
+  // `contentBlock.getEntityKeyAt(leafNode)` and in many cases the leafNode does
+  // not have an entityKey. In those cases the entityKey will be null or
+  // undefined. That's why `getEntityKeyAt()` is typed to return `?string`.
+  // See https://github.com/facebook/draft-js/blob/2da3dcb1c4c106d1b2a0f07b3d0275b8d724e777/src/model/immutable/BlockNode.js#L51
+  entityKey: ?string,
+
   key: React.Key,
   offsetKey: string,
   start: number,