Fix warnings and errors detected by dart2js.

test/observe_test.dart

 // Copyright (c) 2012, 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.
 
 /** Tests for some of the utility helper functions used by the compiler. */
 library observe_test;
 
 import 'dart:collection' show LinkedHashMap;
 import 'package:unittest/compact_vm_config.dart';
 import 'package:unittest/unittest.dart';
 import 'package:web_ui/observe.dart';
-import 'package:web_ui/observe/observable.dart' show hasObservers;
+import 'package:web_ui/observe/observable.dart' show hasObservers, observeReads;
 import 'package:web_ui/src/utils_observe.dart' show setImmediate;
 
 main() {
   useCompactVMConfiguration();
 
   group('ObservableReference', () {
     test('no observers', () {
       var t = new ObservableReference<int>(123);
       expect(t.value, 123);
       t.value = 42;
@@ skipping 847 matching lines @@
     var c = _change(123, 43);
     expect(a.hashCode, b.hashCode, reason: 'a == b');
     expect(b.hashCode, isNot(equals(c.hashCode)), reason: 'b != c');
   });
 }
 
 _change(oldValue, newValue) => new ChangeNotification(oldValue, newValue);
 
 _record(type, key, oldValue, newValue) =>
     new ChangeRecord(type, key, oldValue, newValue);