Revert "Move toString() to collection classes."

tests/corelib/set_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";
-
-void main() {
-  Set s = new HashSet();
-  s.add(1);
-  Expect.equals("{1}", s.toString());
-  s.remove(1);
-  s.add(s);
-  Expect.equals("{{...}}", s.toString());
-
-  Queue q = new ListQueue(4);
-  q.add(1);
-  q.add(2);
-  q.add(q);
-  q.add(s);
-
-  Expect.equals("{1, 2, {...}, {{...}}}", q.toString());
-
-  // Throwing in the middle of a toString does not leave the
-  // set as being visited
-  q.addLast(new ThrowOnToString());
-  Expect.throws(q.toString, (e) => e == "Bad!");
-  q.removeLast();
-  Expect.equals("{1, 2, {...}, {{...}}}", q.toString());
-}
-
-class ThrowOnToString {
-
-  String toString() {
-    throw "Bad!";
-  }
-}