Fixed strong mode errors and warnings reachable from lib/observe.dart

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.
 
 library observe.src.change_notifier;
 
 import 'dart:async';
 import 'dart:collection' show UnmodifiableListView;
 import 'package:observe/observe.dart';
 import 'package:observe/src/observable.dart' show notifyPropertyChangeHelper;
 
 /// Mixin and base class for implementing an [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 ChangeNotifier implements Observable {
-  StreamController _changes;
+  StreamController<List<ChangeRecord>> _changes;
   List<ChangeRecord> _records;
 
   Stream<List<ChangeRecord>> get changes {
     if (_changes == null) {
-      _changes = new StreamController.broadcast(sync: true,
-          onListen: observed, onCancel: unobserved);
+      _changes = new StreamController.broadcast(
+          sync: true, onListen: observed, onCancel: unobserved);
     }
     return _changes.stream;
   }
 
   // TODO(jmesserly): should these be public? They're useful lifecycle methods
   // for subclasses. Ideally they'd be protected.
   /// Override this method to be called when the [changes] are first observed.
   void observed() {}
 
   /// Override this method to be called when the [changes] are no longer being
@@ skipping 23 matching lines @@
   /// equal, no change will be recorded.
   ///
   /// For convenience this returns [newValue]. This makes it easy to use in a
   /// setter:
   ///
   ///     var _myField;
   ///     @reflectable get myField => _myField;
   ///     @reflectable set myField(value) {
   ///       _myField = notifyPropertyChange(#myField, _myField, value);
   ///     }
-  notifyPropertyChange(Symbol field, Object oldValue, Object newValue)
-      => notifyPropertyChangeHelper(this, field, oldValue, newValue);
+  /*=T*/ notifyPropertyChange /*<T>*/ (
+          Symbol field, /*=T*/ oldValue, /*=T*/ newValue) =>
+      notifyPropertyChangeHelper /*<T>*/ (this, field, oldValue, newValue);
 
   void notifyChange(ChangeRecord record) {
     if (!hasObservers) return;
 
     if (_records == null) {
       _records = [];
       scheduleMicrotask(deliverChanges);
     }
     _records.add(record);
   }
 }