Add List.asMap().

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 516d0b2d811606bee061d85100949ca39a111054..560f08fe0766941312f5522f11ed53d6a4e1e570 100644
--- a/sdk/lib/core/list.dart
+++ b/sdk/lib/core/list.dart
@@ -257,4 +257,12 @@ abstract class List<E> implements Collection<E> {
    * [start] is greater than the length of the list.
    */
   void insertRange(int start, int length, [E fill]);
+
+  /**
+   * Returns an unmodifiable [Map] view of `this`.
+   *
+   * It has the indices of this list as keys, and the corresponding elements
+   * as values.
+   */
+  Map<int, E> asMap();
 }