add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/lib/src/observable.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;
 
 /**
- * Use `@observable` to make a field automatically observable.
- */
-const ObservableProperty observable = const ObservableProperty();
-
-/**
  * Interface representing an observable object. This is used by data in
  * model-view architectures to notify interested parties of [changes].
  *
  * This object does not require any specific technique to implement
  * observability. If you mixin [ObservableMixin], [dirtyCheck] will know to
  * check for changes on the object. You may also implement change notification
  * yourself, by calling [notifyChange].
  *
  * You can use [ObservableBase] or [ObservableMixin] to implement this.
  */
@@ skipping 187 matching lines @@
 _notifyPropertyChange(Observable obj, Symbol field, Object oldValue,
     Object newValue) {
 
   // TODO(jmesserly): should this be == instead of identical, to prevent
   // spurious loops?
   if (obj.hasObservers && !identical(oldValue, newValue)) {
     obj.notifyChange(new PropertyChangeRecord(field));
   }
   return newValue;
 }
-
-
-/**
- * An annotation that is used to make a property observable.
- * Normally this is used via the [observable] constant, for example:
- *
- *     class Monster {
- *       @observable int health;
- *     }
- *
- * If needed, you can subclass this to create another annotation that will also
- * be treated as observable.
- */
-class ObservableProperty {
-  const ObservableProperty();
-}