Remove explicit interface versions of dartdoc files.

lib/html/doc/interface/Storage.dartdoc

 // 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.
 
 /// @domName Storage
 /**
  * The type used by the
  * [Window.localStorage] and [Window.sessionStorage] properties.
  * Storage is implemented as a Map<String, String>.
  *
@@ skipping 11 matching lines @@
  *     window.localStorage.clear();
  *     assert(window.localStorage.length == 0);
  *
  * For more examples of using this API, see
  * [localstorage_test.dart](http://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/tests/html/localstorage_test.dart).
  * For details on using the Map API, see the
  * [Maps](http://www.dartlang.org/docs/library-tour/#maps-aka-dictionaries-or-hashes)
  * section of the library tour.
  *
  */
-interface Storage extends Map<String, String> {
+abstract class Storage implements Map<String, String> {
 
   /** @domName Storage.length */
-  final int $dom_length;
+  abstract int get $dom_length;
 
   /** @domName Storage.clear */
   void $dom_clear();
 
   /** @domName Storage.getItem */
   String $dom_getItem(String key);
 
   /** @domName Storage.key */
   String $dom_key(int index);
 
   /** @domName Storage.removeItem */
   void $dom_removeItem(String key);
 
   /** @domName Storage.setItem */
   void $dom_setItem(String key, String data);
 
 }