Revert revision 42497 "Remove unittest and matcher from the repo."

This broke 80% of the bots


git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42498 260f80e4-7a28-3924-810f-c04153c831b5
Add Isolate.current getter, returning the current isolate as an Isolate object.

R=floitsch@google.com

Review URL: https://codereview.chromium.org//771993003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42499 260f80e4-7a28-3924-810f-c04153c831b5