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

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 7d0ff160c9f3647831f773ca9058c11d14aee955..f9d3a3f5ed66880912c2b6a78d1b3273b20a8e06 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -95,6 +95,13 @@ abstract class Iterable<E> {
 
   /**
    * 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);
 
@@ -164,7 +171,13 @@ abstract class Iterable<E> {
   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();