Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_UIEvent.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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   // In JS, canBubble and cancelable are technically required parameters to
   // init*Event. In practice, though, if they aren't provided they simply
   // default to false (since that's Boolean(undefined)).
   //
   // Contrary to JS, we default canBubble and cancelable to true, since that's
   // what people want most of the time anyway.
   factory $CLASSNAME(String type,
       {Window view, int detail: 0, bool canBubble: true,
       bool cancelable: true}) {
     if (view == null) {
       view = window;
     }
-    final e = document._createEvent("UIEvent");
+    UIEvent e = document._createEvent("UIEvent");
     e._initUIEvent(type, canBubble, cancelable, view, detail);
     return e;
   }
 $!MEMBERS
 }