feat(transpiler): class fields for Dart

modules/test_lib/src/test_lib.dart

@@ -6,6 +6,8 @@ import 'package:unittest/unittest.dart' hide expect;
 import 'dart:mirrors';
 import 'dart:async';
 
+bool IS_DARTIUM = true;
+
 Expect expect(actual, [matcher]) {
   final expect = new Expect(actual);
   if (matcher != null) expect.to(matcher);

modules/test_lib/src/test_lib.es6

@@ -7,6 +7,7 @@ export var iit = window.iit;
 export var beforeEach = window.beforeEach;
 export var afterEach = window.afterEach;
 export var expect = window.expect;
+export var IS_DARTIUM = false;
 
 // To make testing consistent between dart and js
 window.print = function(msg) {