Work around issue 17083 in pkg/stack_trace.

This also properly reports timer_impl.dart as being in dart:io rather
than dart:async.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//180363003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@33039 260f80e4-7a28-3924-810f-c04153c831b5