flowify a few more Libraries

Libraries/Animation/POPAnimationMixin.js

@@ -11,14 +11,18 @@
  */
 'use strict';
 
-var POPAnimation = require('POPAnimation');
+var POPAnimationOrNull = require('POPAnimation');
 
-if (!POPAnimation) {
+if (!POPAnimationOrNull) {
   // POP animation isn't available in the OSS fork - this is a temporary
   // workaround to enable its availability to be determined at runtime.
   module.exports = (null : ?{});
 } else {
 
+// At this point, POPAnimationOrNull is guaranteed to be
+// non-null. Bring it local to preserve type refinement.
+var POPAnimation = POPAnimationOrNull;
+
 var invariant = require('invariant');
 var warning = require('warning');