Change Set.toSet to always return a set with the same behavior.

sdk/lib/core/iterable.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;
 
 /**
  * An object that uses an [Iterator] to serve objects one at a time.
  *
  * You can iterate over all objects served by an Iterable object
@@ skipping 77 matching lines @@
    * The resulting Iterable runs through the elements returned
    * by [f] for each element of this, in order.
    *
    * The returned [Iterable] is lazy, and calls [f] for each element
    * of this every time it's iterated.
    */
   Iterable expand(Iterable f(E element));
 
   /**
    * Returns true if the collection contains an element equal to [element].
+   *
+   * The equality used to determine wheter [element] is equal to an element of
+   * the iterable, depends on the type of iterable.
+   * For example, a [Set] may have a custom equality
+   * (see, e.g., [Set.identical]) that its `contains` uses.
+   * Likewise the `Iterable` returned by a [Map.keys] call
+   * will likely use the same equality that the `Map` uses for keys.
    */
   bool contains(Object element);
 
   /**
    * Applies the function [f] to each element of this collection.
    */
   void forEach(void f(E element));
 
   /**
    * Reduces a collection to a single value by iteratively combining elements
@@ skipping 49 matching lines @@
 
   /**
    * Creates a [List] containing the elements of this [Iterable].
    *
    * The elements are in iteration order. The list is fixed-length
    * if [growable] is false.
    */
   List<E> toList({ bool growable: true });
 
   /**
-   * Creates a [Set] containing the elements of this [Iterable].
+   * Creates a [Set] containing the same elements as this iterable.
+   *
+   * The returned `Set` will have the same `Set.length`
+   * as the `length` of this iterable,
+   * and its `Set.contains` will return the same result
+   * as the `contains` of this iterable.
+   * The order of the elements may be different.
    */
   Set<E> toSet();
 
   /**
    * Returns the number of elements in [this].
    *
    * Counting all elements may be involve running through all elements and can
    * therefore be slow.
    */
   int get length;
@@ skipping 175 matching lines @@
  */
 abstract class BidirectionalIterator<E> implements Iterator<E> {
   /**
    * Move back to the previous element.
    *
    * Returns true and updates [current] if successful. Returns false
    * and sets [current] to null if there is no previous element.
    */
   bool movePrevious();
 }