Support is/as expressions in analyzer2dart.

BUG=
R=paulberry@google.com, sigurdm@google.com

Review URL: https://codereview.chromium.org//693153002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41438 260f80e4-7a28-3924-810f-c04153c831b5