Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_CompositionEvent.darttemplate

 // Copyright (c) 2013, 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 {
   factory $CLASSNAME(String type,
       {bool canBubble: false, bool cancelable: false, Window view,
       String data, String locale}) {
     if (view == null) {
       view = window;
     }
-    var e = document._createEvent("CompositionEvent");
+    CompositionEvent e = document._createEvent("CompositionEvent");
 
 $if DART2JS
     if (Device.isFirefox) {
       // Firefox requires the locale parameter that isn't supported elsewhere.
       JS('void', '#.initCompositionEvent(#, #, #, #, #, #)',
           e, type, canBubble, cancelable, view, data, locale);
     } else {
       e._initCompositionEvent(type, canBubble, cancelable, view, data);
     }
 $else
       e._initCompositionEvent(type, canBubble, cancelable, view, data);
 $endif
 
     return e;
   }
 
 $!MEMBERS
 }