Issue 21375. Fix for FunctionTypeAliasElement-based FunctionType source.

R=brianwilkerson@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=21375

Review URL: https://codereview.chromium.org//672013002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41254 260f80e4-7a28-3924-810f-c04153c831b5