Migrating another batch of tests to dart 2.

tests/language_strong/closure_with_super_field_test.dart

-// Copyright (c) 2012, 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.
-
-import "package:expect/expect.dart";
-import "package:meta/meta.dart" show virtual;
-
-class A {
-  @virtual
-  int a;
-  A() : a = 42;
-}
-
-class B extends A {
-  int get a => 54;
-  returnSuper() => super.a;
-  returnSuperInClosure() => () => super.a;
-}
-
-main() {
-  B b = new B();
-  Expect.equals(54, b.a);
-  Expect.equals(42, b.returnSuper());
-  Expect.equals(42, b.returnSuperInClosure()());
-}