feat(runner): support running dart2js spec files

This commit supports running Dart2JS output in NodeJS.  Officially,
Dart2JS in supposed to only generate code for running in a real
webbrowser.  With this patch, the dart2js code can also be executed in
NodeJS.

Ref: https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/sdk/lib/js/dart2js/js_dart2js.dart?spec=svn32943&r=32943#487