feat(core, testability): PendingAsync service

A new PendingAsync service that is used to track pending async
operations in Angular.  VmTurnZone ties in to this service to track
timers as async tasks and correctly handles canceled timers.  Http
registers async tasks for network requests.  The Testability API uses
the new API to provide a much more useful whenStable implementation.
Why do we have to increment it here?