Introduce a class encapsulating arguments descriptor arrays.

runtime/vm/native_arguments.h

Index: runtime/vm/native_arguments.h
diff --git a/runtime/vm/native_arguments.h b/runtime/vm/native_arguments.h
index 211f04a458bbf57320140f3019cc5a5ec7b2b4f0..7111c86e3fd2260269e51829097ab10c533e97c6 100644
--- a/runtime/vm/native_arguments.h
+++ b/runtime/vm/native_arguments.h
@@ -50,8 +50,8 @@ void SetReturnValueHelper(Dart_NativeArguments, Dart_Handle);
 // following signature:
 //   void function_name(NativeArguments arguments);
 // Inside the function, arguments are accessed as follows:
-//   const Instance& arg0 = Instance::CheckedHandle(arguments.At(0));
-//   const Smi& arg1 = Smi::CheckedHandle(arguments.At(1));
+//   const Instance& arg0 = Instance::CheckedHandle(arguments.ArgAt(0));
+//   const Smi& arg1 = Smi::CheckedHandle(arguments.ArgAt(1));
 // The return value is set as follows:
 //   arguments.SetReturn(result);
 // NOTE: Since we pass 'this' as a pass-by-value argument in the stubs we don't