This also caps the log transcript size.

BUG=https://code.google.com/p/dart/issues/detail?id=15547
R=nweiz@google.com

Review URL: https://codereview.chromium.org//111103002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@31072 260f80e4-7a28-3924-810f-c04153c831b5