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

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index ce24d01a7258989997626ad3831062e093a305b4..6dcad8db1106a21deb79de2dfaece4baf2e7f6e4 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -652,6 +652,8 @@ class JavaScriptBackend extends Backend {
   ClassElement jsBoolClass;
 
   ClassElement jsIndexableClass;
+  ClassElement jsMutableIndexableClass;
+
   ClassElement jsMutableArrayClass;
   ClassElement jsFixedArrayClass;
   ClassElement jsExtendableArrayClass;
@@ -898,6 +900,8 @@ class JavaScriptBackend extends Backend {
 
     jsIndexableClass =
         compiler.findInterceptor(const SourceString('JSIndexable'));
+    jsMutableIndexableClass =
+        compiler.findInterceptor(const SourceString('JSMutableIndexable'));
 
     // TODO(kasperl): Some tests do not define the special JSArray
     // subclasses, so we check to see if they are defined before