Migrated test block 17 to Dart 2.0.

tests/corelib_2/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;
@@ skipping 12 matching lines @@
   m[err] = 1;
   Expect.throws(m.toString, (e) => e == "Bad!");
   m.remove(err);
 }
 
 class ThrowOnToString {
   String toString() {
     throw "Bad!";
   }
 }