Issue 20827. Add imports as needed the 'Add type annotation' Quick Assist.

I left some TODOs, the CL is big enough already.

R=paulberry@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=20827

Review URL: https://codereview.chromium.org//706263003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41620 260f80e4-7a28-3924-810f-c04153c831b5