Set MethodInvocation.interfaceTarget during type inference.

pkg/front_end/testcases/inference/future_then_downwards_method_target.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';
 
 main() {
   Future<int> f;
   Future<List<int>>
       b = /*info:ASSIGNMENT_CAST should be pass*/ /*@promotedType=none*/ f
-          . /*@typeArgs=List<dynamic>*/ then(
+          . /*@typeArgs=List<dynamic>*/ /*@target=Future::then*/ then(
               /*@returnType=List<dynamic>*/ (/*@type=int*/ x) => /*@typeArgs=dynamic*/ [])
-          .whenComplete(/*@returnType=Null*/ () {});
-  b = /*@promotedType=none*/ f. /*@typeArgs=List<int>*/ then(
-      /*@returnType=List<int>*/ (/*@type=int*/ x) => /*@typeArgs=int*/ []);
+          . /*@target=Future::whenComplete*/ whenComplete(
+              /*@returnType=Null*/ () {});
+  b = /*@promotedType=none*/ f
+      . /*@typeArgs=List<int>*/ /*@target=Future::then*/ then(
+          /*@returnType=List<int>*/ (/*@type=int*/ x) => /*@typeArgs=int*/ []);
 }