Migrated test block 17 to Dart 2.0.

tests/corelib/map_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";
-
-void main() {
-  Map m = new Map();
-
-  m[0] = 0;
-  m[1] = 1;
-  m[2] = m;
-
-  Expect.equals('{0: 0, 1: 1, 2: {...}}', m.toString());
-
-  // Throwing in the middle of a toString does not leave the
-  // map as being visited
-  ThrowOnToString err = new ThrowOnToString();
-  m[1] = err;
-  Expect.throws(m.toString, (e) => e == "Bad!");
-  m[1] = 1;
-  Expect.equals('{0: 0, 1: 1, 2: {...}}', m.toString());
-  m[err] = 1;
-  Expect.throws(m.toString, (e) => e == "Bad!");
-  m.remove(err);
-}
-
-class ThrowOnToString {
-  String toString() {
-    throw "Bad!";
-  }
-}