Partially migrate from old getter syntax to new one.

lib/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.
 
 class _EventsImpl implements Events {
   /* Raw event target. */
   // TODO(jacobr): it would be nice if we could specify this as
   // _EventTargetImpl or EventTarget
   final Dynamic _ptr;
 
@@ skipping 38 matching lines @@
   }
 
   void _remove(EventListener listener, bool useCapture) {
     _ptr.$dom_removeEventListener(_type, listener, useCapture);
   }
 }
 
 
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 
-  Events get on() => new _EventsImpl(this);
+  Events get on => new _EventsImpl(this);
 $!MEMBERS
 }