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

Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js

Index: Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js
diff --git a/Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js b/Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js
index 64b6a15a92af12d6375d45f69a11840076a00f1b..dacf39e44aa778743ac71144cca574b3e1e19b64 100644
--- a/Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js
+++ b/Source/devtools/front_end/ui_lazy/TimelineOverviewPane.js
@@ -141,7 +141,17 @@ WebInspector.TimelineOverviewPane.prototype = {
     {
         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);
 
         /**