polymer: fix analyzer warning

pkg/polymer/lib/src/events.dart

 // 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.
 
 /// Code from declaration/events.js
 part of polymer;
 
 /// An extension of [polymer_expressions.PolymerExpressions] that adds support
 /// for binding events using `on-eventName` using [PolymerEventBindings].
 // TODO(jmesserly): the JS layering is a bit odd, with polymer-dev implementing
@@ skipping 74 matching lines @@
       var sub = node.on[eventType].listen(handler);
 
       if (oneTime) return null;
       return new _EventBindable(sub, path);
     };
   }
 }
 
 
 class _EventBindable extends Bindable {
-  final StreamSubscription _sub;
+  StreamSubscription _sub;
   final String _path;
 
   _EventBindable(this._sub, this._path);
 
   // TODO(rafaelw): This is really pointless work. Aside from the cost
   // of these allocations, NodeBind is going to setAttribute back to its
   // current value. Fixing this would mean changing the TemplateBinding
   // binding delegate API.
   get value => '{{ $_path }}';
 
@@ skipping 45 matching lines @@
   return map;
 }();
 
 // Dart note: we need this function because we have additional renames JS does
 // not have. The JS renames are simply case differences, whereas we have ones
 // like doubleclick -> dblclick and stripping the webkit prefix.
 String _eventNameFromType(String eventType) {
   final result = _reverseEventTranslations[eventType];
   return result != null ? result : eventType;
 }