Fix a few strong mode errors in dart:html

tools/dom/templates/html/impl/impl_EventTarget.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;
 
 /**
  * Base class that supports listening for and dispatching browser events.
  *
  * Normally events are accessed via the Stream getter:
@@ skipping 34 matching lines @@
   final EventTarget _ptr;
 
   Events(this._ptr);
 
   Stream operator [](String type) {
     return new _EventStream(_ptr, type, false);
   }
 }
 
 class ElementEvents extends Events {
-  /* Raw event target. */
-  final Element _ptr;
   static final webkitEvents = {
-    'animationend' : 'webkitAnimationEnd', 
-    'animationiteration' : 'webkitAnimationIteration', 
-    'animationstart' : 'webkitAnimationStart', 
-    'fullscreenchange' : 'webkitfullscreenchange', 
+    'animationend' : 'webkitAnimationEnd',
+    'animationiteration' : 'webkitAnimationIteration',
+    'animationstart' : 'webkitAnimationStart',
+    'fullscreenchange' : 'webkitfullscreenchange',
     'fullscreenerror' : 'webkitfullscreenerror',
-    'keyadded' : 'webkitkeyadded', 
-    'keyerror' : 'webkitkeyerror', 
-    'keymessage' : 'webkitkeymessage', 
-    'needkey' : 'webkitneedkey', 
-    'pointerlockchange' : 'webkitpointerlockchange', 
-    'pointerlockerror' : 'webkitpointerlockerror', 
-    'resourcetimingbufferfull' : 'webkitresourcetimingbufferfull', 
+    'keyadded' : 'webkitkeyadded',
+    'keyerror' : 'webkitkeyerror',
+    'keymessage' : 'webkitkeymessage',
+    'needkey' : 'webkitneedkey',
+    'pointerlockchange' : 'webkitpointerlockchange',
+    'pointerlockerror' : 'webkitpointerlockerror',
+    'resourcetimingbufferfull' : 'webkitresourcetimingbufferfull',
     'transitionend': 'webkitTransitionEnd',
     'speechchange' : 'webkitSpeechChange'
   };
 
-  ElementEvents(Element ptr) : this._ptr = ptr, super(ptr);
+  ElementEvents(Element ptr) : super(ptr);
 
   Stream operator [](String type) {
     if (webkitEvents.keys.contains(type.toLowerCase())) {
       if (Device.isWebKit) {
         return new _ElementEventStreamImpl(
             _ptr, webkitEvents[type.toLowerCase()], false);
       }
     }
     return new _ElementEventStreamImpl(_ptr, type, false);
   }
@@ skipping 35 matching lines @@
     // TODO(leafp): This check is avoid a bug in our dispatch code when
     // listener is null.  The browser treats this call as a no-op in this
     // case, so it's fine to short-circuit it, but we should not have to.
     if (listener != null) {
       _removeEventListener(type, listener, useCapture);
     }
   }
 
 $!MEMBERS
 }