cleanup: Use Function.prototype.bind()

When not using arrow notation with anonymous functions, we use Lang.bind()
to bind `this` to named callbacks. However since ES5, this functionality
is already provided by Function.prototype.bind() - in fact, Lang.bind()
itself uses it when no extra arguments are specified. Just use the built-in
function directly where possible, and use arrow notation in the few places
where we pass additional arguments.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/23

js/ui/extensionDownloader.js

@@ -195,11 +195,11 @@ var InstallExtensionDialog = new Lang.Class({
         this._invocation = invocation;
 
         this.setButtons([{ label: _("Cancel"),
-                           action: Lang.bind(this, this._onCancelButtonPressed),
+                           action: this._onCancelButtonPressed.bind(this),
                            key:    Clutter.Escape
                          },
                          { label:  _("Install"),
-                           action: Lang.bind(this, this._onInstallButtonPressed),
+                           action: this._onInstallButtonPressed.bind(this),
                            default: true
                          }]);