Change "typedef" to "class" in core library and related tests.

pkg/observe/lib/src/change_notifier.dart

 // Copyright (c) 2013, 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.
 
 part of observe;
 
 /**
  * Base class implementing [Observable] object that performs its own change
  * notifications, and does not need to be considered by [Observable.dirtyCheck].
  *
  * When a field, property, or indexable item is changed, a derived class should
  * call [notifyPropertyChange]. See that method for an example.
  */
-typedef ChangeNotifierBase = Object with ChangeNotifierMixin;
+class ChangeNotifierBase = Object with ChangeNotifierMixin;
 
 /**
  * Mixin implementing [Observable] object that performs its own change
  * notifications, and does not need to be considered by [Observable.dirtyCheck].
  *
  * When a field, property, or indexable item is changed, a derived class should
  * call [notifyPropertyChange]. See that method for an example.
  */
 abstract class ChangeNotifierMixin implements Observable {
   StreamController _changes;
@@ skipping 57 matching lines @@
   void notifyChange(ChangeRecord record) {
     if (!hasObservers) return;
 
     if (_records == null) {
       _records = [];
       scheduleMicrotask(deliverChanges);
     }
     _records.add(record);
   }
 }