Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_Event.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
@@ skipping 15 matching lines @@
    * Normally events should be created via their constructors, if available.
    *
    *     var e = new Event.type('MouseEvent', 'mousedown', true, true);
    */
   factory Event.eventType(String type, String name, {bool canBubble: true,
       bool cancelable: true}) {
     final Event e = document._createEvent(type);
     e._initEvent(name, canBubble, cancelable);
     return e;
   }
-  
+
   /** The CSS selector involved with event delegation. */
   String _selector;
 
   /**
    * A pointer to the element whose CSS selector matched within which an event
    * was fired. If this Event was not associated with any Event delegation,
    * accessing this value will throw an [UnsupportedError].
    */
   Element get matchingTarget {
     if (_selector == null) {
       throw new UnsupportedError('Cannot call matchingTarget if this Event did'
           ' not arise as a result of event delegation.');
     }
-    var currentTarget = this.currentTarget;
-    var target = this.target;
+    Element currentTarget = this.currentTarget;
+    Element target = this.target;
     var matchedTarget;
     do {
       if (target.matches(_selector)) return target;
       target = target.parent;
     } while (target != null && target != currentTarget.parent);
     throw new StateError('No selector matched for populating matchedTarget.');
   }
 $!MEMBERS
 }