Allow using native JS []= on typed data lists.

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 8d7fe061161427a42fc0972fcb84bc92b820a5d2..2d73dc91acd0333c377df4f7f17dc557290c1ea9 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_array.dart
@@ -294,6 +294,6 @@ class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
  * Dummy subclasses that allow the backend to track more precise
  * information about arrays through their type.
  */
-class JSMutableArray extends JSArray {}
+class JSMutableArray extends JSArray implements JSMutableIndexable {}

[kasperl, 2013/04/24 10:43:03]

I'll see if I can remove this in the next CL.

 class JSFixedArray extends JSMutableArray {}
 class JSExtendableArray extends JSMutableArray {}