add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/polymer/lib/deserialize.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.
 
 library polymer.deserialize;
 
 import 'dart:convert' show JSON;
-import 'dart:mirrors' show reflect, TypeMirror;
+import 'dart:mirrors' show TypeMirror;
 
 final _typeHandlers = () {
   // TODO(jmesserly): switch to map and symbol literal form when supported.
   var m = new Map();
   m[#dart.core.String] = (x, _) => x;
   m[#dart.core.Null] = (x, _) => x;
   m[#dart.core.DateTime] = (x, _) {
     // TODO(jmesserly): shouldn't need to try-catch here
     // See: https://code.google.com/p/dart/issues/detail?id=1878
     try {
@@ skipping 23 matching lines @@
     // quotes altogether, parse will fail.
     return JSON.decode(value.replaceAll("'", '"'));
 
     // TODO(jmesserly): deserialized JSON is not assignable to most objects in
     // Dart. We should attempt to convert it appropriately.
   } catch(e) {
     // The object isn't valid JSON, return the raw value
     return value;
   }
 }