Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_union_downwards_deprecated_2.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([x]) {}
-  dynamic noSuchMethod(invocation);
-  MyFuture<S> then<S>(dynamic f(T x), {Function onError}) => null;
-}
-
-MyFuture f;
-// Instantiates Future<int>
-Future<int> t1 = f.then(/*@returnType=MyFuture<int>*/ (/*@type=dynamic*/ _) =>
-    /*@typeArgs=int*/ new MyFuture.value('hi'));
-
-// Instantiates List<int>
-Future<List<int>> t2 = f.then(
-    /*@returnType=List<int>*/ (/*@type=dynamic*/ _) => /*@typeArgs=int*/ [3]);
-Future<List<int>> g2() async {
-  return /*@typeArgs=int*/ [3];
-}
-
-Future<List<int>> g3() async {
-  return /*@typeArgs=List<int>*/ new MyFuture.value(
-      /*@typeArgs=int*/ [3]);
-}