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

Source/devtools/front_end/ui/KeyboardShortcut.js

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2009 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 274 matching lines @@
 WebInspector.KeyboardShortcut._modifiersToString = function(modifiers)
 {
     var isMac = WebInspector.isMac();
     var m = WebInspector.KeyboardShortcut.Modifiers;
     var modifierNames = new Map([
         [m.Ctrl, isMac ? "\u2303" : "Ctrl\u200A+\u200A"],
         [m.Alt, isMac ? "\u2325" : "Alt\u200A+\u200A"],
         [m.Shift, isMac ? "\u21e7" : "Shift\u200A+\u200A"],
         [m.Meta, isMac ? "\u2318" : "Win\u200A+\u200A"]
     ]);
-    return [m.Ctrl, m.Alt, m.Shift, m.Meta].map(m => modifiers & m ? modifierNames.get(m) : "").join("");
+    return [m.Ctrl, m.Alt, m.Shift, m.Meta].map(mapModifiers).join("");
+
+    /**
+     * @param {number} m
+     * @return {string}
+     */
+    function mapModifiers(m)
+    {
+        return modifiers & m ? /** @type {string} */ (modifierNames.get(m)) : "";
+    }
 };
 
 WebInspector.KeyboardShortcut.SelectAll = WebInspector.KeyboardShortcut.makeKey("a", WebInspector.KeyboardShortcut.Modifiers.CtrlOrMeta);