add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer_expressions/test/bindings_test.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 bindings_test;
 
 import 'dart:html';
 import 'package:mdv/mdv.dart' as mdv;
 import 'package:observe/observe.dart';
 import 'package:observe/src/microtask.dart';
@@ skipping 54 matching lines @@
       // But propagating observable values through reassign the value and
       // selection will be preserved.
       expect(el.selectionStart, 4);
       expect(el.selectionEnd, 4);
 
       subscription.cancel();
     }));
   });
 }
 
+@reflectable
 class NotifyModel extends ChangeNotifierBase {
   var _x;
   NotifyModel([this._x]);
 
   get x => _x;
   set x(value) {
     _x = notifyPropertyChange(const Symbol('x'), _x, value);
   }
 }