Fix annotation of generic method invocations in Fasta inference tests.

pkg/front_end/testcases/inference/future_then_upwards_3.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;
 
 import 'dart:async';
 
 class MyFuture<T> implements Future<T> {
   MyFuture() {}
   MyFuture.value(T x) {}
   dynamic noSuchMethod(invocation);
   MyFuture<S> then<S>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
 void main() {
-  var /*@type=Future<double>*/ f =
-      foo().then(/*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3);
+  var /*@type=Future<double>*/ f = foo(). /*@typeArgs=double*/ then(
+      /*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3);
   Future<int> f2 = /*error:INVALID_ASSIGNMENT*/ /*@promotedType=none*/ f;
 
   // The unnecessary cast is to illustrate that we inferred <double> for
   // the generic type args, even though we had a return type context.
-  Future<num> f3 = /*info:UNNECESSARY_CAST*/ foo().then(
+  Future<num> f3 = /*info:UNNECESSARY_CAST*/ foo(). /*@typeArgs=double*/ then(
       /*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3) as Future<double>;
 }
 
 Future foo() => new Future<int>.value(1);