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

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 8b276c076080bd7e208c3e8de1f21ea7ab287ec3..1fbbeee8b5d890edb2aa6bf0c501c2b53ff1c946 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -15,6 +15,11 @@ part of dart.core;
  *
  * 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();