Add EfficientLength marker interface to some iterabels.

sdk/lib/core/set.dart

Index: sdk/lib/core/set.dart
diff --git a/sdk/lib/core/set.dart b/sdk/lib/core/set.dart
index 19be402b71fe579abfc4191ea6faa4d3299dfaba..998fe98261a3356ddb3ff14d62a46414b9052247 100644
--- a/sdk/lib/core/set.dart
+++ b/sdk/lib/core/set.dart
@@ -20,7 +20,7 @@ part of dart.core;
  * 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].
    *
@@ -48,6 +48,14 @@ abstract class Set<E> extends IterableBase<E> {
   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);