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/compiler/runtime_metadata_spec.ts

@@ -72,8 +72,7 @@ export function main() {
          inject([RuntimeMetadataResolver], (resolver: RuntimeMetadataResolver) => {
            var value: string =
                resolver.getDirectiveMetadata(ComponentWithoutModuleId).type.moduleUrl;
-           var expectedEndValue =
-               IS_DART ? 'base/dist/dart/angular2/test/compiler/runtime_metadata_spec.dart' : './';
+           var expectedEndValue = IS_DART ? 'test/compiler/runtime_metadata_spec.dart' : './';
            expect(value.endsWith(expectedEndValue)).toBe(true);
          }));
     });