Add EfficientLength marker interface to some iterabels.

sdk/lib/core/map.dart

Index: sdk/lib/core/map.dart
diff --git a/sdk/lib/core/map.dart b/sdk/lib/core/map.dart
index 38cb7cf3bf5e0a947ad659543a2d7ae3585f325e..eb6c6769ce8f18cdceba0133cbdf5d8298003844 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -153,16 +153,28 @@ abstract class Map<K, V> {
 
   /**
    * The keys of [this].
+   *
+   * The `contains` method of the returned iterable is as efficient as
+   * [containsKey].
+   *
+   * The returned iterable is guaranteed to have the same `length` as this map,
+   * and its `length` getter is efficient.
    */
   Iterable<K> get keys;
 
   /**
    * The values of [this].
+   *
+   * This iterable is guaranteed to have the same `length` as this map,
+   * and its `length` getter is efficient.
    */
   Iterable<V> get values;
 
   /**
    * The number of {key, value} pairs in the map.
+   *
+   * This operation is efficient, and doesn't require iterating and counting
+   * the entries.
    */
   int get length;