Convert gr-comment-api-mock to class-based element

Change-Id: If0ad1b84a6bc3c93578cab903986f295af5f0ec7

polygerrit-ui/app/elements/diff/gr-comment-api/gr-comment-api-mock.js

@@ -17,16 +17,20 @@
 (function() {
   'use strict';
 
-  Polymer({
-    is: 'comment-api-mock',
+  class CommentApiMock extends Polymer.GestureEventListeners(
+      Polymer.LegacyElementMixin(
+          Polymer.Element)) {
+    static get is() { return 'comment-api-mock'; }
 
-    properties: {
-      _changeComments: Object,
-    },
+    static get properties() {
+      return {
+        _changeComments: Object,
+      };
+    }
 
     loadComments() {
       return this._reloadComments();
-    },
+    }
 
     /**
      * For the purposes of the mock, _reloadDrafts is not included because its
@@ -38,13 +42,15 @@
       return this._reloadComments().then(() => {
         return e.detail.resolve();
       });
-    },
+    }
 
     _reloadComments() {
       return this.$.commentAPI.loadAll(this._changeNum)
           .then(comments => {
             this._changeComments = this.$.commentAPI._changeComments;
           });
-    },
-  });
+    }
+  }
+
+  customElements.define(CommentApiMock.is, CommentApiMock);
 })();