Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_then_explicit_future.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";
 
 m1() {
   Future<int> f;
-  var /*@type=Future<List<int>>*/ x = /*@promotedType=none*/ f
-      .then<Future<List<int>>>(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/
-          /*@returnType=List<int>*/ (/*@type=int*/ x) => /*@typeArgs=int*/ []);
+  var /*@type=Future<List<int>>*/ x = /*@promotedType=none*/ f.then<
+          Future<List<int>>>(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/
+      /*@returnType=List<dynamic>*/ (/*@type=int*/ x) => /*@typeArgs=dynamic*/ []);
   Future<List<int>> y = /*@promotedType=none*/ x;
 }
 
 m2() {
   Future<int> f;
   var /*@type=Future<List<int>>*/ x = /*@promotedType=none*/ f.then<List<int>>(
       /*@returnType=List<int>*/ (/*@type=int*/ x) => /*@typeArgs=int*/ []);
   Future<List<int>> y = /*@promotedType=none*/ x;
 }