Fix issue 20983 (https://code.google.com/p/dart/issues/detail?id=20983)

The check for load error on completion of async load was very inefficient.
Added a Hash set to ensure we don't repeatedly traverse through a library
when trying to establish transitive load errors.

R=iposva@google.com

Review URL: https://codereview.chromium.org//573293004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40422 260f80e4-7a28-3924-810f-c04153c831b5
Support for breakpoints in isolates.

It mostly works modulo https://code.google.com/p/dart/issues/detail?id=20974 (extra paused event),
which looks as a VM bug.

R=devoncarew@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=9209

Review URL: https://codereview.chromium.org//577003004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40423 260f80e4-7a28-3924-810f-c04153c831b5