Ignore ButtonType when computing an extracted method return type.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//789543002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42201 260f80e4-7a28-3924-810f-c04153c831b5
Issue 21801. Index redirecting constructor invocations.

R=brianwilkerson@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=21801

Review URL: https://codereview.chromium.org//783223003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42202 260f80e4-7a28-3924-810f-c04153c831b5