feat(runner): support dart2js version 1.6.0-dev.0.0

This add to commit 876a3c04c07a9f8d97e1edca3ec1f76e51e1a310

dart2js now refers to the global object as "self" (as a step toward "use
strict" since Primitives.computeGlobalThis() wouldn't work in strict
mode.)  So we now publish "self" to the global namespace to support dart
version 1.6.0-dev.0.0.

Refs:
- File: https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/sdk/lib/js/dart2js/js_dart2js.dart?r=37483#100
- Revision r37483: https://code.google.com/p/dart/source/detail?r=37483
- Review: https://codereview.chromium.org//345533004
- Diff: https://code.google.com/p/dart/source/diff?r=37483&format=side&path=/branches/bleeding_edge/dart/sdk/lib/js/dart2js/js_dart2js.dart#sc_svn37483_100

      - final JsObject context = _wrapToDart(Primitives.computeGlobalThis());
      + final JsObject context = _wrapToDart(JS('', 'self'));
release v0.23.4
feat(clientsidescripts): use testability API for everything
release v0.23.5
release v0.23.6