Change the toString method of IterableBase/IterableMixin to show some elements.

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());
+  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());
+  Expect.equals("(1, 2, (...), {{...}})", q.toString());
 }
 
 class ThrowOnToString {
 
   String toString() {
     throw "Bad!";
   }
 }