Adding 'part of html' to all files which are used by dart:html.

sdk/lib/html/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;
+
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME(String type, Window view, int detail, int screenX,
       int screenY, int clientX, int clientY, int button, [bool canBubble = true,
       bool cancelable = true, bool ctrlKey = false, bool altKey = false,
       bool shiftKey = false, bool metaKey = false,
       EventTarget relatedTarget = null]) =>
       _$(CLASSNAME)FactoryProvider.create$CLASSNAME(
           type, view, detail, screenX, screenY,
           clientX, clientY, button, canBubble, cancelable,
           ctrlKey, altKey, shiftKey, metaKey,
@@ skipping 21 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;
     }
   }
 }