Add List.asMap().

sdk/lib/_internal/compiler/implementation/lib/js_array.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
index d99e16c304c790a393c516f6081d27eba18ad548..9b73f4080cbd0388225a53463aec53ec2e9cfde1 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
@@ -297,4 +297,8 @@ class JSArray<E> implements List<E> {
     if (index >= length || index < 0) throw new RangeError.value(index);
     JS('void', r'#[#] = #', this, index, value);
   }
+
+  Map<int, E> asMap() {
+    return IterableMixinWorkaround.asMapList(this);
+  }
 }