Fix exception when running Sheriff-o-Matic tests.

Tools/GardeningServer/ui/test/ct-commit-list-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-commit-list.html">
 
 <link rel="import" href="../../model/ct-commit-log-mock.html">
 
@@ skipping 12 matching lines @@
 
   describe('commit list UI', function() {
     beforeEach(function(done) {
       var repos = [
           { commits: [ new CTCommitMock(), new CTCommitMock() ],
             expanded: false },
           { commits: [ new CTCommitMock(), new CTCommitMock() ],
             expanded: false }
       ];
 
-      var CommitList = Object.create(Object);
-      CommitList.prototype.repositories = function() {
-        return repos;
+      list.commitList = {
+        repositories: function() {
+          return repos;
+        }
       };
-      list.commitList = CommitList;
 
       setTimeout(done);
     });
 
     it('should show no commits by default', function() {
       var commits = list.shadowRoot.querySelectorAll('ct-commit');
       assert.lengthOf(commits, 0);
     });
 
     describe('expanded test', function() {
       beforeEach(function(done) {
         list.commitList.repositories().first().expanded = true;
         list.commitList.repositories().last().expanded = true;
         setTimeout(done);
       });
 
       it('should show commits when expanded', function() {
         assert.lengthOf(list.shadowRoot.querySelectorAll('ct-commit'), 4);
       });
     });
   });
 });
 
 })()
 </script>