Fix native argument handling

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 14922)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -3939,39 +3939,18 @@
 DART_EXPORT Dart_Handle Dart_GetNativeArgument(Dart_NativeArguments args,
                                                int index) {
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
-  const int apparent_count = Dart_GetNativeArgumentCount(args);
-  if ((index < 0) || (index >= apparent_count)) {
+  if ((index < 0) || (index >= arguments->NativeArgCount())) {
     return Api::NewError(
         "%s: argument 'index' out of range. Expected 0..%d but saw %d.",
-        CURRENT_FUNC, apparent_count - 1, index);
+        CURRENT_FUNC, arguments->NativeArgCount() - 1, index);
   }
-  // Hide closure object if present.
-  index += arguments->Count() - apparent_count;
-
-  Isolate* isolate = arguments->isolate();
-  CHECK_ISOLATE(isolate);
-  if (index == 0) {
-    if (arguments->ToInstanceFunction() && arguments->ToClosureFunction()) {
-      // Retrieve the receiver from the context.
-      const Context& context = Context::Handle(isolate->top_context());
-      return Api::NewHandle(isolate, context.At(0));
-    }
-  }
-  return Api::NewHandle(isolate, arguments->At(index));
+  return Api::NewHandle(arguments->isolate(), arguments->NativeArgAt(index));
 }
 
 
 DART_EXPORT int Dart_GetNativeArgumentCount(Dart_NativeArguments args) {
   NativeArguments* arguments = reinterpret_cast<NativeArguments*>(args);
-  int count = arguments->Count();
-  // In the instance closure function case, the receiver is accessed from
-  // the context and the closure at index 0 is hidden, so the apparent
-  // argument count remains unchanged.
-  if (arguments->ToClosureFunction() && !arguments->ToInstanceFunction()) {
-    // The closure at index 0 is hidden and therefore not counted.
-    count--;
-  }
-  return count;
+  return arguments->NativeArgCount();
 }