Port of github.com/polymer/polymer.

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 Object observable = const _ObservableAnnotation();
+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].
  *
@@ skipping 96 matching lines @@
     var values = new Map<Symbol, Object>();
 
     // Note: we scan for @observable regardless of whether the base type
     // actually includes this mixin. While perhaps too inclusive, it lets us
     // avoid complex logic that walks "with" and "implements" clauses.
     for (var type = mirror.type; type != _objectType; type = type.superclass) {
       for (var field in type.variables.values) {
         if (field.isFinal || field.isStatic || field.isPrivate) continue;
 
         for (var meta in field.metadata) {
-          if (identical(observable, meta.reflectee)) {
+          if (meta.reflectee is ObservableProperty) {
             var name = field.simpleName;
             // Note: since this is a field, getting the value shouldn't execute
             // user code, so we don't need to worry about errors.
             values[name] = mirror.getField(name).reflectee;
             break;
           }
         }
       }
     }
 
@@ skipping 75 matching lines @@
   // 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;
 }
 
 
 /**
- * The type of the `@observable` annotation.
+ * An annotation that is used to make a property observable.
+ * Normally this is used via the [observable] constant, for example:
  *
- * Library private because you should be able to use the [observable] field
- * to get the one and only instance. We could make it public though, if anyone
- * needs it for some reason.
+ *     class Monster {
+ *       @observable int health;
+ *     }
+ *
+ * If needed, you can subclass this to create another annotation that will also
+ * be treated as observable.
  */
-class _ObservableAnnotation {
-  const _ObservableAnnotation();
+class ObservableProperty {
+  const ObservableProperty();
 }