Add EfficientLength marker interface to some iterabels.

sdk/lib/core/list.dart

Index: sdk/lib/core/list.dart
diff --git a/sdk/lib/core/list.dart b/sdk/lib/core/list.dart
index 868e9537175a276e7f36dbc6462e8277e8176cf0..eca42d828a4a888a4b6102668d33ce88a0ad5d19 100644
--- a/sdk/lib/core/list.dart
+++ b/sdk/lib/core/list.dart
@@ -36,7 +36,7 @@ part of dart.core;
  * temporarily and is restored before continuing the iteration, the iterator
  * does not detect it.
  */
-abstract class List<E> implements Iterable<E> {
+abstract class List<E> implements Iterable<E>, EfficientLength {
   /**
    * Creates a list of the given length.
    *
@@ -128,6 +128,9 @@ abstract class List<E> implements Iterable<E> {
    * Returns the number of objects in this list.
    *
    * The valid indices for a list are `0` through `length - 1`.
+   *
+   * This operations is efficient, and does not require iterating and counting
+   * the elements.
    */
   int get length;