Check for duplicate library names

test/browser/runtime_tests.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 var assert = chai.assert;
+var core = dart.import('dart/core');
+var collection = dart.import('dart/collection');
 
 suite('generic', () => {
   "use strict";
 
   let generic = dart.generic;
 
   test('zero arguments is not allowed', () => {
     assert.throws(() => { generic(function(){}); });
   });
 
@@ skipping 686 matching lines @@
 
 suite('primitives', function() {
   'use strict';
 
   test('fixed length list', () => {
     let list = new core.List(10);
     list[0] = 42;
     assert.throws(() => list.add(42));
   });
 });