add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer_expressions/lib/src/globals.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.
 
 /**
  * Contains functions that are included by default in [PolymerExpressions].
  *
  *   - [enumerate]: a convenient way to iterate over items and the indexes.
  */
 // Code from https://github.com/google/quiver-dart/commit/52edc4baf37e99ff6a8f99c648b29b135fc0b880
 library polymer_expressions.src.globals;
 
 import 'dart:collection';
+import 'package:observe/observe.dart' show reflectable;
 
 /**
  * Returns an [Iterable] of [IndexedValue]s where the nth value holds the nth
  * element of [iterable] and its index.
  */
 Iterable<IndexedValue> enumerate(Iterable iterable) =>
     new EnumerateIterable(iterable);
 
-class IndexedValue<V> {
+@reflectable class IndexedValue<V> {
   final int index;
   final V value;
 
   IndexedValue(this.index, this.value);
 }
 
 /**
  * An [Iterable] of [IndexedValue]s where the nth value holds the nth
  * element of [iterable] and its index. See [enumerate].
  */
@@ skipping 30 matching lines @@
 
   bool moveNext() {
     if (_iterator.moveNext()) {
       _current = new IndexedValue(_index++, _iterator.current);
       return true;
     }
     _current = null;
     return false;
   }
 }