Fix annotation of generic method invocations in Fasta inference tests.

pkg/front_end/testcases/inference/generic_methods_basic_downward_inference.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /*@testedFeatures=inference*/
 library test;
 
 T f<S, T>(S s) => null;
 main() {
-  String x = f(42);
+  String x = /*@typeArgs=int, String*/ f(42);
   String y = (f)(42);
 }