Add second argument to Future error handlers.
Add second argument to Future error handlers.

R=lrn@google.com, nweiz@google.com

Review URL: https://codereview.chromium.org//25027004

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@28510 260f80e4-7a28-3924-810f-c04153c831b5

computmaxer/pub,dart-lang/pub,maxwellpeterson-wf/pub,cjkao/dart-pub,dart-lang/pub