Migrating a block of dart 1 tests to dart 2

tests/language_2/closure2_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // Dart test for closures.
 
 import "package:expect/expect.dart";
 
 bounce(fn) {
   return fn();
 }
 
 demo(s) {
   var i, a = bounce(() => s);
   return a;
 }
 
 main() {
   Expect.equals("Bounce!", demo("Bounce!"));
 }