Work around issue 14125 in stack_trace by adding a dummy setter.

R=rnystrom@google.com
BUG=13945

Review URL: https://codereview.chromium.org//27416002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@28811 260f80e4-7a28-3924-810f-c04153c831b5