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

sdk/lib/collection/list.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.
 
 part of dart.collection;
 
+/** A reusable set used to identify cyclic lists during toString() calls. */
+Set _toStringVisiting = new HashSet.identity();
+
 /**
  * Abstract implementation of a list.
  *
  * All operations are defined in terms of `length`, `operator[]`,
  * `operator[]=` and `length=`, which need to be implemented.
  */
 typedef ListBase<E> = Object with ListMixin<E>;
 
 /**
  * Base implementation of a [List] class.
  *
  * This class can be used as a mixin.
  *
  * This implements all read operations using only the `length` and
  * `operator[]` members. It implements write operations using those and
  * `length=` and `operator[]=`
  */
 abstract class ListMixin<E> implements List<E> {
-  // A list to identify cyclic lists during toString() calls.
-  static List _toStringList = new List();
 
   // Iterable interface.
   Iterator<E> get iterator => new ListIterator<E>(this);
 
   E elementAt(int index) => this[index];
 
   void forEach(void action(E element)) {
     int length = this.length;
     for (int i = 0; i < length; i++) {
       action(this[i]);
@@ skipping 443 matching lines @@
     } else {
       for (E element in iterable) {
         this[index++] = element;
       }
     }
   }
 
   Iterable<E> get reversed => new ReversedListIterable(this);
 
   String toString() {
-    for (int i = 0; i < _toStringList.length; i++) {
-      if (identical(_toStringList[i], this)) { return '[...]'; }
+    if (_toStringVisiting.contains(this)) {
+      return '[...]';
     }
 
     var result = new StringBuffer();
     try {
-      _toStringList.add(this);
+      _toStringVisiting.add(this);
       result.write('[');
       result.writeAll(this, ', ');
       result.write(']');
      } finally {
-       assert(identical(_toStringList.last, this));
-       _toStringList.removeLast();
+       _toStringVisiting.remove(this);
      }
 
     return result.toString();
   }
 }