Make Iterable more powerful (and lazy).

sdk/lib/core/set.dart

 // Copyright (c) 2011, 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.
 
 /**
  * This class is the public interface of a set. A set is a collection
  * without duplicates.
  */
 abstract class Set<E> extends Collection<E> {
   factory Set() => new _HashSetImpl<E>();
@@ skipping 47 matching lines @@
    * Returns a new set which is the intersection between this set and
    * the given collection.
    */
   Set<E> intersection(Collection<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 
+  /**
+   * Returns an immutable view of [this].
+   *
+   * The returned [Set] is backed by [this] but cannot change [this].
+   */
+  Set<E> get view;
 }
 
 abstract class HashSet<E> extends Set<E> {
   factory HashSet() => new _HashSetImpl<E>();
 
   /**
    * Creates a [Set] that contains all elements of [other].
    */
   factory HashSet.from(Iterable<E> other) => new _HashSetImpl<E>.from(other);
 }
@@ skipping 160 matching lines @@
   }
 
   // The entries in the set. May contain null or the sentinel value.
   List<E> _entries;
 
   // The next valid index in [_entries] or the length of [entries_].
   // If it is the length of [_entries], calling [hasNext] on the
   // iterator will return false.
   int _nextValidIndex;
 }