Removed polymer & mirror from Observatory

runtime/observatory/tests/observatory_ui/nav/library-menu/element_test.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:observatory/src/elements/nav/library_menu.dart';
 import '../../mocks.dart';
 
 main() {
   NavLibraryMenuElement.tag.ensureRegistration();
 
   final i_ref = const IsolateRefMock(id: 'i-id', name: 'i-name');
   final l_ref = const LibraryRefMock(id: 'l-id', name: 'l-name');
   test('instantiation', () {
     final e = new NavLibraryMenuElement(i_ref, l_ref);
     expect(e, isNotNull, reason: 'element correctly created');
     expect(e.isolate, equals(i_ref));
     expect(e.library, equals(l_ref));
   });
   test('elements created after attachment', () async {
     final e = new NavLibraryMenuElement(i_ref, l_ref);
     document.body.append(e);
     await e.onRendered.first;
-    expect(e.shadowRoot.children.length, isNonZero, reason: 'has elements');
+    expect(e.children.length, isNonZero, reason: 'has elements');
     e.remove();
     await e.onRendered.first;
-    expect(e.shadowRoot.children.length, isZero, reason: 'is empty');
+    expect(e.children.length, isZero, reason: 'is empty');
   });
 }