add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/test/path_observer_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.
 
 import 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 import 'observe_test_utils.dart';
 
 // This file contains code ported from:
 // https://github.com/rafaelw/ChangeSummary/blob/master/tests/test.js
@@ skipping 206 matching lines @@
     performMicrotaskCheckpoint();
     expect(values, [1, 2]);
 
     sub.cancel();
     model[#a] = 3;
     performMicrotaskCheckpoint();
     expect(values, [1, 2]);
   });
 }
 
+@reflectable
 class TestModel extends ChangeNotifierBase {
   var _a, _b, _c;
 
   TestModel();
 
   get a => _a;
 
   void set a(newValue) {
     _a = notifyPropertyChange(#a, _a, newValue);
   }
@@ skipping 13 matching lines @@
 
 class WatcherModel extends ObservableBase {
   // TODO(jmesserly): dart2js does not let these be on the same line:
   // @observable var a, b, c;
   @observable var a;
   @observable var b;
   @observable var c;
 
   WatcherModel();
 }