Fixing custom event test on IE

For some reason, Event.preventDefault on IE is not triggering defaultPrevented back at the call site.
BUG=
R=vsm@google.com

Review URL: https://codereview.chromium.org//27536010

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@28767 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed