Turn ReactDOMFiberComponent into a singleton

And turn on Flow.

src/renderers/dom/fiber/wrappers/ReactDOMFiberInput.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactDOMFiberInput
+ * @flow
  */
 
 'use strict';

src/renderers/dom/fiber/wrappers/ReactDOMFiberOption.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactDOMFiberOption
+ * @flow
  */
 
 'use strict';

src/renderers/dom/fiber/wrappers/ReactDOMFiberSelect.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactDOMFiberSelect
+ * @flow
  */
 
 'use strict';

src/renderers/dom/fiber/wrappers/ReactDOMFiberTextarea.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactDOMFiberTextarea
+ * @flow
  */
 
 'use strict';