React.Element<any> / React.Element<*> -> React.Node as much as possible

Summary:
Most of the time when people annotate something with `React.Element<any>` or `React.Element<*>` they don't really care about it really being a React element, and would be just as happy with `React.Node`. The exceptions I encountered when writing this diff were:

1) places where people do `React.cloneElement` -- in that case we of course need an element
2) components that want some of the props or children to be a certain component type -- but in those cases we ought to specify the type of element rather than accepting `any` or `*`

As such, it's reasonable to replace `React.Element<any>` and `React.Element<*>` if doing so does not cause any Flow errors, and this should put a noticeable dent in our usage of unsafe Flow types.

Reviewed By: camspiers, stryju

Differential Revision: D7810983

fbshipit-source-id: d28f645a63cdfbae74370817e36c5bf590f543d6

Author: miorel

src/component/base/DraftEditor.react.js

@@ -286,7 +286,7 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
     );
   }
 
-  _renderPlaceholder(): ?React.Element<any> {
+  _renderPlaceholder(): React.Node {
     if (this._showPlaceholder()) {
       const placeHolderProps = {
         text: nullthrows(this.props.placeholder),

src/component/contents/DraftEditorBlock.react.js

@@ -134,7 +134,7 @@ class DraftEditorBlock extends React.Component<Props> {
     }
   }
 
-  _renderChildren(): Array<React.Element<any>> {
+  _renderChildren(): Array<React.Node> {
     const block = this.props.block;
     const blockKey = block.getKey();
     const text = block.getText();

src/model/decorators/DraftDecorator.js

@@ -53,7 +53,7 @@ export type DraftDecorator = {
  */
 export type DraftDecoratorComponentProps = {
   blockKey: BlockNodeKey,
-  children?: Array<React.Element<any>>,
+  children?: Array<React.Node>,
   contentState: ContentState,
   decoratedText: string,
   dir: ?HTMLDir,

src/model/immutable/DraftBlockRenderConfig.js

@@ -14,6 +14,6 @@
 
 export type DraftBlockRenderConfig = {
   element: string,
-  wrapper?: React$Element<any>,
+  wrapper?: React.Node,
   aliasedElements?: Array<string>,
 };