Fix warnings and errors detected by dart2js.

lib/observe.dart

 // Copyright (c) 2012, 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.
 
 /**
  * A library for observing changes to observable Dart objects.
  * Similar in spirit to EcmaScript Harmony
  * [Object.observe](http://wiki.ecmascript.org/doku.php?id=harmony:observe), but
  * able to observe expressions and not just objects, so long as the expressions
  * are computed from observable objects.
  *
  * See the `observable` annotation and the `observe` function.
  */
 // Note: one intentional difference from Harmony Object.observe is that our
 // change batches are tracked on a per-observed expression basis, instead of
 // per-observer basis.
 // We do this because there is no cheap way to store data on a Dart
 // function (Expando uses linear search on the VM: http://dartbug.com/7558).
 // This difference means that a given observer will be called with one batch of
 // changes for each object it is observing.
 // TODO(jmesserly): this behavior is not ideal. It's really powerful to be able
 // to get all changes in one "transaction". We should look at alternate
 // approaches.
-library observe;
+library web_ui.observe;
 
 export 'observe/list.dart';
 export 'observe/map.dart';
 export 'observe/reference.dart';
 export 'observe/set.dart';
 
 export 'observe/observable.dart'
     // Hide methods that are only used when implementing Observable:
     hide hasObservers, observeReads, notifyChange, notifyRead;