Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_MutationEvent.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,
       {bool canBubble: false, bool cancelable: false, Node relatedNode,
       String prevValue, String newValue, String attrName, int attrChange: 0}) {
 
-    var event = document._createEvent('MutationEvent');
+    MutationEvent event = document._createEvent('MutationEvent');
     event._initMutationEvent(type, canBubble, cancelable, relatedNode,
         prevValue, newValue, attrName, attrChange);
     return event;
   }
 $!MEMBERS
 }