add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/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.
 
 part of observe;
 
 /**
  * Base class implementing [Observable] object that performs its own change
  * notifications, and does not need to be considered by [Observable.dirtyCheck].
  *
@@ skipping 53 matching lines @@
   /**
    * Notify that the field [name] of this object has been changed.
    *
    * The [oldValue] and [newValue] are also recorded. If the two values are
    * identical, no change will be recorded.
    *
    * For convenience this returns [newValue]. This makes it easy to use in a
    * setter:
    *
    *     var _myField;
-   *     get myField => _myField;
-   *     set myField(value) {
+   *     @reflectable get myField => _myField;
+   *     @reflectable set myField(value) {
    *       _myField = notifyPropertyChange(#myField, _myField, value);
    *     }
    */
   notifyPropertyChange(Symbol field, Object oldValue, Object newValue)
       => _notifyPropertyChange(this, field, oldValue, newValue);
 
   void notifyChange(ChangeRecord record) {
     if (!hasObservers) return;
 
     if (_records == null) {
       _records = [];
       scheduleMicrotask(deliverChanges);
     }
     _records.add(record);
   }
 }