Add EfficientLength marker interface to some iterabels.

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.
 
 part of dart.core;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
  * to be in the set, or to _not_ be in the set.
  *
  * Set implementations may consider some elements indistinguishable. These
  * elements are treated as being the same for any operation on the set.
  *
  * The default `Set` implementation, [HashSet], considers objects
  * indistinguishable if they are equal with regard to [Object.operator==].
  *
  * Sets may be either ordered or unordered. [HashSet] is unordered and doesn't
  * guarantee anything about the order that elements are accessed in by
  * iteration. [LinkedHashSet] iterates in the insertion order of its elements.
  */
-abstract class Set<E> extends IterableBase<E> {
+abstract class Set<E> extends IterableBase<E> implements EfficientLength {
   /**
    * Creates an empty [Set].
    *
    * The created `Set` is a [LinkedHashSet]. As such, it considers elements that
    * are equal (using `==`) to be indistinguishable, and requires them to
    * have a compatible [Object.hashCode] implementation.
    */
   factory Set() = LinkedHashSet<E>;
 
   /**
    * Creates an empty identity [Set].
    *
    * The created `Set` is a [LinkedHashSet] that uses identity as equality
    * relation.
    */
   factory Set.identity() = LinkedHashSet<E>.identity;
 
   /**
    * Creates a [Set] that contains all elements of [other].
    *
    * The created `Set` is a [HashSet]. As such, it considers elements that
    * are equal (using `==`) to be undistinguishable, and requires them to
    * have a compatible [Object.hashCode] implementation.
    */
   factory Set.from(Iterable<E> other) = LinkedHashSet<E>.from;
 
   /**
+   * Returns the number of elements in this set.
+   *
+   * This operation is efficient, and does not require iterating and counting
+   * the elements.
+   */
+  int get length;
+
+  /**
    * Returns true if [value] is in the set.
    */
   bool contains(Object value);
 
   /**
    * Adds [value] into the set.
    *
    * The method has no effect if [value] is already in the set.
    */
   void add(E value);
@@ skipping 60 matching lines @@
    * That is, the returned set contains all the elements of this `Set` that
    * are not elements of [other].
    */
   Set<E> difference(Set<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 }