Report HintCode.UNUSED_FIELD

There are two positives in dart2js, valid as I can see.
There are several positives in analyzer and analysis_server.

R=brianwilkerson@google.com
BUG=

Review URL: https://codereview.chromium.org//728503003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41727 260f80e4-7a28-3924-810f-c04153c831b5
Specialize _toPow2String for bigint to make it linear instead of quadratic.
Remove some duplicated bigint corelib tests.
Cleanup corelib tests status file.
Partially fix issue 20879 (not closed).

R=zra@google.com

Review URL: https://codereview.chromium.org//723963006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41728 260f80e4-7a28-3924-810f-c04153c831b5