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

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

Index: sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index d363676fc67c67bca60a2f4abefc93617b661c1b..2dd0780d53669c36b6d279820929cb067e1db33f 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -19,7 +19,7 @@ import 'dart:_interceptors' show getInterceptor,
                                  makeDispatchRecord,
                                  setDispatchProperty,
                                  Interceptor,
-                                 JSIndexable;
+                                 JSMutableIndexable;
 import "dart:_collection-dev" as _symbol_dev;
 
 part 'constant_map.dart';
@@ -1302,7 +1302,7 @@ malformedTypeCast(value, type, reasons) {
  * objects that support integer indexing. This interface is not
  * visible to anyone, and is only injected into special libraries.
  */
-abstract class JavaScriptIndexingBehavior extends JSIndexable {
+abstract class JavaScriptIndexingBehavior extends JSMutableIndexable {
 }
 
 // TODO(lrn): These exceptions should be implemented in core.