Fix flow errors (#6719)

The new flow somehow found these on my machine but nowhere else
and not previously.
(cherry picked from commit 069f8099d6)

src/renderers/native/ReactNativeDOMIDOperations.js

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

src/renderers/native/ReactNativeMount.js

@@ -175,7 +175,7 @@ var ReactNativeMount = {
    * @param {View} view View tree image.
    * @param {number} containerViewID View to insert sub-view into.
    */
-  _mountImageIntoNode: function(mountImage, containerID) {
+  _mountImageIntoNode: function(mountImage : number, containerID : number) {
     // Since we now know that the `mountImage` has been mounted, we can
     // mark it as such.
     var childTag = mountImage;