Make Bigint instances immutable by removing all setters.
This required rewriting a large portion of bigint.dart.

R=rmacnak@google.com

Review URL: https://codereview.chromium.org//842033005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@43492 260f80e4-7a28-3924-810f-c04153c831b5
13 files changed