add test to ensure unlisten events do not fire

test/unit/events.html

@@ -170,6 +170,11 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
           var fn = el._recallEventHandler(el, 'foo', el, 'missing');
           assert.ok(fn, 'should be cached');
         });
+
+        test('once unlistened, no handler fire', function() {
+          el.fire('foo');
+          assert.equal(el._warned.length, 1, 'event should not be handled anymore')
+        });
       });
     });
   </script>