Add default implementation of toString for ListMixin.

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());
+}