chore(test): migrate Dart tests to package:test

Instead of running with karma and the karma-dart shim, run dart
tests directly using the new package:test runner. This migrates
away from package:unittest.

Fixes a couple tests, mostly associated with depending on absolute
URLs or editing the test providers after an injector had already
been created.

Remove karma-dart and associated files. Change gupfiles to run tests
via `pub run test` instead.

modules/angular2/test/platform/dom/events/event_manager_spec.ts

@@ -18,10 +18,10 @@ import {DOM} from 'angular2/src/platform/dom/dom_adapter';
 export function main() {
   var domEventPlugin;
 
-  beforeEach(() => { domEventPlugin = new DomEventsPlugin(); });
-
   describe('EventManager', () => {
 
+    beforeEach(() => { domEventPlugin = new DomEventsPlugin(); });
+
     it('should delegate event bindings to plugins that are passed in from the most generic one to the most specific one',
        () => {
          var element = el('<div></div>');