[cupertino_icons] Adds a 'library' directive after the library comment

Author: foxtrotravi

third_party/packages/cupertino_icons/test/cupertino_icons_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 /// This test file is primarily here to serve as a source for code excerpts.
+library;
 
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';