Renaming dart:html library from 'html' to 'dart.html'

tools/dom/templates/html/dart2js/impl_MouseEvent.darttemplate

 // 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.
 
-part of html;
+part of $LIBRARYNAME;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME(String type,
       {Window view, int detail: 0, int screenX: 0, int screenY: 0,
       int clientX: 0, int clientY: 0, int button: 0, bool canBubble: true,
       bool cancelable: true, bool ctrlKey: false, bool altKey: false,
       bool shiftKey: false, bool metaKey: false, EventTarget relatedTarget}) {
 
     if (view == null) {
       view = window;
@@ skipping 27 matching lines @@
       // Firefox does not support offsetY.
       var target = this.target;
       if (!(target is Element)) {
         throw new UnsupportedError(
             'offsetY is only supported on elements');
       }
       return this.clientY - this.target.getBoundingClientRect().top;
     }
   }
 }