Update urls for polymer

pkg/observe/lib/observe.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.
 
 /**
  * *Warning*: this library is experimental, and APIs are subject to change.
  *
  * This library is used to observe changes to [Observable] types. It also
  * has helpers to make implementing and using [Observable] objects easy.
  *
@@ skipping 47 matching lines @@
  *       var obj = new Monster();
  *       obj.changes.listen((records) {
  *         print('Changes to $obj were: $records');
  *       });
  *       // Schedules asynchronous delivery of these changes
  *       obj.damage(10);
  *       obj.damage(20);
  *       print('done!');
  *     }
  *
- * [Tools](https://github.com/dart-lang/web-ui) exist to convert the first form
- * into the second form automatically, to get the best of both worlds.
+ * [Tools](https://www.dartlang.org/polymer-dart/) exist to convert the first
+ * form into the second form automatically, to get the best of both worlds.
  */
 library observe;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 
 // Note: this is an internal library so we can import it from tests.
 // TODO(jmesserly): ideally we could import this with a prefix, but it caused
 // strange problems on the VM when I tested out the dirty-checking example
 // above.
 import 'src/dirty_check.dart';
 
 part 'src/bind_property.dart';
 part 'src/change_notifier.dart';
 part 'src/change_record.dart';
 part 'src/compound_binding.dart';
 part 'src/list_path_observer.dart';
 part 'src/observable.dart';
 part 'src/observable_box.dart';
 part 'src/observable_list.dart';
 part 'src/observable_map.dart';
 part 'src/path_observer.dart';
 part 'src/to_observable.dart';