Add Set.union, Set.difference. Change Set methods to expect Set.

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;
 
 /**
  * The [Iterable] interface allows to get an [Iterator] out of an
  * [Iterable] object.
  *
  * This interface is used by the for-in construct to iterate over an
  * [Iterable] object.
  * The for-in construct takes an [Iterable] object at the right-hand
  * side, and calls its [iterator] method to get an [Iterator] on it.
  *
  * A user-defined class that implements the [Iterable] interface can
  * be used as the right-hand side of a for-in construct.
+ *
+ * Iterating over an iterable should not have side effects.
+ * If someone receives an iterable, they should be free to iterate it more
+ * than once (for example once to get the length, and then once to do something

[floitsch, 2013/03/12 16:25:00]

Doesn't fit this CL.
Also I don't necessarily agree.

Maybe: Iterables may have side effects when iterating over them (for example as
a result of `map`). However, it is bad style to send them to other
(unsuspecting) functions. In general it should be reasonable to iterate over
iterables several times. For example, once to get the length, and once more to
do something for each element.

[Lasse Reichstein Nielsen, 2013/03/13 13:01:42]

Removed for now.

+ * for each element).
  */
 abstract class Iterable<E> {
   const Iterable();
 
   /**
    * Create an [Iterable] that generates its elements dynamically.
    *
    * The [Iterators] created by the [Iterable] will count from
    * zero to [:count - 1:] while iterating, and call [generator]
    * with that index to create the next value.
@@ skipping 405 matching lines @@
  */
 abstract class BidirectionalIterator<T> extends Iterator<T> {
   /**
    * 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();
 }