Enable strip-types transform for internal code

Flow is coming so we need to be ready.

Somebody started writing a diff adding types to keyMirror which doesn't build. So I tested that this does:

```diff
diff --git a/src/utils/keyMirror.js b/src/utils/keyMirror.js
index 0e0c614..eaaa5f0 100644
--- a/src/utils/keyMirror.js
+++ b/src/utils/keyMirror.js
@@ -32,7 +32,11 @@ var invariant = require('invariant');
  * @param {object} obj
  * @return {object}
  */
-var keyMirror = function(obj) {
+function keyMirror<T>(obj: T): T {
+ return keyMirrorInternal(obj);
+}
+
+function keyMirrorInternal(obj) {
   var ret = {};
   var key;
   invariant(
```