refactor: Remove isDart from public API

BREAKING CHANGE:

- `IS_DARTIUM` no longer exported

modules/angular2/src/test_lib/test_lib.dart

@@ -28,8 +28,6 @@ import 'package:angular2/src/facade/collection.dart' show StringMapWrapper;
 import 'test_injector.dart';
 export 'test_injector.dart' show inject;
 
-bool IS_DARTIUM = true;
-
 List _testBindings = [];
 Injector _injector;
 bool _isCurrentTestAsync;

modules/angular2/src/test_lib/test_lib.ts

@@ -33,9 +33,6 @@ export interface NgMatchers extends jasmine.Matchers {
 
 export var expect: (actual: any) => NgMatchers = <any>_global.expect;
 
-// TODO vsavkin: remove it and use lang/isDart instead
-export var IS_DARTIUM = false;
-
 export class AsyncTestCompleter {
   _done: Function;