Issue 19141. Update 'pause on exception' in all live isolates on preference change.

R=devoncarew@google.com
BUG= https://code.google.com/p/dart/issues/detail?id=19141

Review URL: https://codereview.chromium.org//607893008

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40746 260f80e4-7a28-3924-810f-c04153c831b5