test(logging.html): #5007 make sure _logger called one time

I really don't think its useful/necessary

test/unit/logging.html

@@ -88,16 +88,17 @@
   suite('_logger', () => {
     test('_log with single parameter', () => {
       const msg = 'log test';
-      let logedMsg;
+      const logHistory = [];
       const orgConsole = console;      
       window.console = {
         log: msg => {
-          logedMsg = msg;
+          logHistory.push(msg);
         }
       };
       Polymer.Base._log(msg);
       window.console = orgConsole;
-      assert.equal(msg, logedMsg);
+      assert.equal(logHistory[0], msg);
+      assert.equal(logHistory.length, 1, 'console.log called more than one time');
     });
   });
   </script>