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.

Ref:  [js_dart2js.dart](https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/sdk/lib/js/dart2js/js_dart2js.dart?r=37483#100) @ [r37483](https://code.google.com/p/dart/source/detail?r=37483)  ([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))

```diff
- final JsObject context = _wrapToDart(Primitives.computeGlobalThis());
+ final JsObject context = _wrapToDart(JS('', 'self'));
```