Adding generic methods to the language specification.

Change-Id: I49404e81258fc688caef5c044ff70e52508a4423
Reviewed-on: https://dart-review.googlesource.com/14525
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Reviewed-by: Leaf Petersen <leafp@google.com>
Reviewed-by: Régis Crelier <regis@google.com>