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());
+}