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

lib/src/change_record.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_record;
 
 import 'package:observe/observe.dart';
 
-
 /// Records a change to an [Observable].
 // TODO(jmesserly): remove this type
 abstract class ChangeRecord {}
 
 /// A change record to a field of an observable object.
 class PropertyChangeRecord<T> extends ChangeRecord {
   /// The object that changed.
   final object;
 
   /// The name of the property that changed.
   final Symbol name;
 
   /// The previous value of the property.
   final T oldValue;
 
   /// The new value of the property.
   final T newValue;
 
   PropertyChangeRecord(this.object, this.name, this.oldValue, this.newValue);
 
   String toString() =>
       '#<PropertyChangeRecord $name from: $oldValue to: $newValue>';
 }