Fix a few strong mode errors in 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 $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   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;
     }
-    var event = document._createEvent('MouseEvent');
+    MouseEvent event = document._createEvent('MouseEvent');
     event._initMouseEvent(type, canBubble, cancelable, view, detail,
         screenX, screenY, clientX, clientY, ctrlKey, altKey, shiftKey, metaKey,
         button, relatedTarget);
     return event;
   }
 $!MEMBERS
 
   @DomName('MouseEvent.clientX')
   @DomName('MouseEvent.clientY')
   Point get client => new Point(_clientX, _clientY);
@@ skipping 34 matching lines @@
   Point get screen => new Point(_screenX, _screenY);
 
   @DomName('MouseEvent.layerX')
   @DomName('MouseEvent.layerY')
   Point get layer => new Point(_layerX, _layerY);
 
   @DomName('MouseEvent.pageX')
   @DomName('MouseEvent.pageY')
   Point get page => new Point(_pageX, _pageY);
 }