Fix exception when running Sheriff-o-Matic tests.

Tools/GardeningServer/ui/test/ct-results-panel-tests.html

 <!--
 Copyright 2014 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <link rel="import" href="../ct-results-panel.html">
 
 <link rel="import" href="../../model/ct-failure.html">
 
@@ skipping 79 matching lines @@
     177164, 177165
   ),
 ];
 
 describe('ct-results-panel', function() {
   var panel;
   var failures;
 
   beforeEach(function(done) {
     panel = document.createElement('ct-results-panel');
+    panel.hidden = true;
+    document.body.appendChild(panel);
+
     if (failures)
       panel.failures = failures;
 
     setTimeout(done);
   });
 
+  afterEach(function() {
+    document.body.removeChild(panel);
+  });
+
   describe('empty', function() {
     before(function() {
       failures = undefined;
     });
 
     it('should show no results', function() {
       var message = panel.shadowRoot.querySelector('.message');
       assert.equal(message.textContent, 'No results to display.');
     });
   });
@@ skipping 50 matching lines @@
       assert.lengthOf(results, 1);
       assert.equal(results[0].failure, failures[0]);
 
       assert.lengthOf(panel.shadowRoot.querySelectorAll('ct-embedded-flakiness-dashboard'), 0);
     });
   });
 });
 
 })();
 </script>