Only one real listener per listen call

Actually use event listener cache correctly

Fixes #2534 Polymer Gestures creates a new bound handler for every `listen` call

src/standard/events.html

@@ -80,8 +80,12 @@
      * @param {string} methodName Name of handler method on `this` to call.
      */
     listen: function(node, eventName, methodName) {
-      this._listen(node, eventName,
-        this._createEventHandler(node, eventName, methodName));
+      var handler = this._recallEventHandler(this, eventName, node, methodName);
+      // reuse cache'd handler
+      if (!handler) {
+        handler = this._createEventHandler(node, eventName, methodName);
+      }
+      this._listen(node, eventName, handler);
     },
 
     _boundListenerKey: function(eventName, methodName) {
@@ -140,6 +144,7 @@
      anymore.
      */
     unlisten: function(node, eventName, methodName) {
+      // leave handler in map for cache purposes
       var handler = this._recallEventHandler(this, eventName, node, methodName);
       if (handler) {
         this._unlisten(node, eventName, handler);