Migrated test block 15 to Dart 2.0.

tests/corelib/list_to_string_test.dart

-// Copyright (c) 2013, 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 'dart:collection';
-
-class MyList extends ListBase {
-  final list;
-  MyList(this.list);
-
-  get length => list.length;
-  set length(val) {
-    list.length = val;
-  }
-
-  operator [](index) => list[index];
-  operator []=(index, val) => list[index] = val;
-}
-
-main() {
-  Expect.equals("[]", [].toString());
-  Expect.equals("[1]", [1].toString());
-  Expect.equals("[1, 2]", [1, 2].toString());
-  Expect.equals("[]", const [].toString());
-  Expect.equals("[1]", const [1].toString());
-  Expect.equals("[1, 2]", const [1, 2].toString());
-  Expect.equals("[]", new MyList([]).toString());
-  Expect.equals("[1]", new MyList([1]).toString());
-  Expect.equals("[1, 2]", new MyList([1, 2]).toString());
-}