fix observe package to be strong mode clean

lib/src/bind_property.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.bind_property;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 
 /// Forwards an observable property from one object to another. For example:
@@ skipping 11 matching lines @@
 ///       String get prop => _otherModel.value;
 ///       set prop(String value) { _otherModel.value = value; }
 ///     }
 ///
 /// See also [notifyPropertyChange].
 // TODO(jmesserly): make this an instance method?
 StreamSubscription onPropertyChange(
     Observable source, Symbol sourceName, void callback()) {
   return source.changes.listen((records) {
     for (var record in records) {
-      if (record is PropertyChangeRecord &&
-          (record as PropertyChangeRecord).name == sourceName) {
+      if (record is PropertyChangeRecord && record.name == sourceName) {
         callback();
         break;
       }
     }
   });
 }