add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/observe/lib/src/metadata.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, or to indicate
+ * that a property is observable.
+ */
+const ObservableProperty observable = const ObservableProperty();
+
+/**
+ * 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.
+ */
+// Note: observable properties imply reflectable.
+class ObservableProperty {
+  const ObservableProperty();
+}
+
+
+/**
+ * Use `@reflectable` to make a type or member available to reflection in the
+ * observe package. This is necessary to make the member visible to
+ * [PathObserver], or similar systems, once the code is deployed.
+ */
+const Reflectable reflectable = const Reflectable();
+
+/**
+ * An annotation that is used to make a type or member reflectable. This makes
+ * it available to [PathObserver] at runtime. For example:
+ *
+ *     @reflectable
+ *     class Monster extends ChangeNotifierBase {
+ *       int _health;
+ *       int get health => _health;
+ *       ...
+ *     }
+ *     ...
+ *       // This will work even if the code has been tree-shaken/minified:
+ *       final monster = new Monster();
+ *       new PathObserver(monster, 'health').changes.listen(...);
+ */
+class Reflectable {
+  const Reflectable();
+}