Set MethodInvocation.interfaceTarget during type inference.

pkg/front_end/testcases/inference/future_or_subtyping.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';
 
 void add(int x) {}
 add2(int y) {}
 main() {
   Future<int> f;
   var /*@type=Future<void>*/ a = /*@promotedType=none*/ f
-      . /*@typeArgs=void*/ then(add);
+      . /*@typeArgs=void*/ /*@target=Future::then*/ then(add);
   var /*@type=Future<dynamic>*/ b = /*@promotedType=none*/ f
-      . /*@typeArgs=dynamic*/ then(add2);
+      . /*@typeArgs=dynamic*/ /*@target=Future::then*/ then(add2);
 }