Private Properties phase 1 (#7666)

* Private Properties phase 1

Co-authored-by: CodingItWrong

* Private fields are optional

* Docs update

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/destructure/output.js

@@ -1,8 +1,8 @@
 class AnchorLink extends Component {
   render() {
-    const _props = this.props,
-          isExternal = _props.isExternal,
-          children = _props.children;
+    const _this$props = this.props,
+          isExternal = _this$props.isExternal,
+          children = _this$props.children;
 
     if (isExternal) {
       return <a>{children}</a>;