DevTools: do not use arrow functions unless they are in stable.

Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 123 matching lines @@
         }
     },
 
     /**
      * @return {!Promise<!DocumentFragment>}
      */
     _buildPopoverContents: function()
     {
         var document = this.element.ownerDocument;
         var x = this._cursorPosition;
-        var promises = this._overviewControls.map(control => control.popoverElementPromise(x));
+        var promises = this._overviewControls.map(mapToPopover);
+
+        /**
+         * @param {!WebInspector.TimelineOverview} control
+         * @return {!Promise<?Element>}
+         */
+        function mapToPopover(control)
+        {
+            return control.popoverElementPromise(x)
+        }
+
         return Promise.all(promises).then(buildFragment);
 
         /**
          * @param {!Array<?Element>} elements
          * @return {!DocumentFragment}
          */
         function buildFragment(elements)
         {
             var fragment = document.createDocumentFragment();
             elements.remove(null);
@@ skipping 467 matching lines @@
     },
 
     resetCanvas: function()
     {
         this._canvas.width = this.element.clientWidth * window.devicePixelRatio;
         this._canvas.height = this.element.clientHeight * window.devicePixelRatio;
     },
 
     __proto__: WebInspector.VBox.prototype
 }