Assert annotations

@jbdeboer @antingshen @pavelgj Could one of you review this PR.

This should really users not to forget to annotate their classes (I got trapped in https://github.com/angular/angular.dart/pull/1395#issuecomment-53662934).

By default the check is enabled only when asserts are enabled then there is no penalty in production mode (which should use the static DI anyway).

/ref https://github.com/angular/di.dart/pull/104 for my former impl on DI v1 which did not support libAnnotations.