- Add custom patching function for repeating callback functions to
support requestAnimationFrame wrapping.
- Bind requestAnimationFrame callback functions to the current zone
unless it is the root zone.
- DRY up requestAnimationFrame spec file (to remove duplication across
browsers)
- Add test that verifies that when registering frame callbacks from
within a frame callback, the zone in which they execute remains
constant.
- Add test to verify the wrapper is tolerant of invalid arguments.
- Add note to readme about behavior of raf

I noticed when including Polymer 1.0 elements in an Angular 2 app, this line throws an error saying it can't find `value` of undefined. I've added a simple workaround which seems to fix the issue and (so far at least) doesn't seem to cause any additional issues with Polymer or Angular.

cc @justinfagnani @addyosmani