Adding generic methods to the language specification.

Change-Id: I49404e81258fc688caef5c044ff70e52508a4423
Reviewed-on: https://dart-review.googlesource.com/14525
Commit-Queue: Erik Ernst <[email protected]>
Reviewed-by: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Leaf Petersen <[email protected]>
Reviewed-by: Régis Crelier <[email protected]>

Author: eernstg