Move observe to observe package

pkg/observe/lib/observe_html.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.
 
 // TODO(jmesserly): can we handle this more elegantly?
 // In general, it seems like we want a convenient way to take a Stream plus a
 // getter and convert this into an Observable.
 
 /** Helpers for exposing dart:html as observable data. */
 library polymer.observe_html;
@@ skipping 43 matching lines @@
 }
 
 /** Bind a CSS class to the observable [object] and property [path]. */
 PathObserver bindCssClass(Element element, String className,
     Observable object, String path) {
 
   return new PathObserver(object, path)..bindSync((value) {
     updateCssClass(element, className, value);
   });
 }