Properly handle errors thrown in [Chain.capture]'s [onError] handler. The Zone infrastructure apparently doesn't automatically pipe errors to the parent Zone (issue 18134). R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//232123003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@34904 260f80e4-7a28-3924-810f-c04153c831b5