Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_union_async_conditional_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;
-}
-
-Future<int> g1(bool x) async {
-  return /*info:DOWN_CAST_COMPOSITE*/ x
-      ? 42
-      : /*@typeArgs=int*/ new MyFuture.value(42);
-}
-
-Future<int> g2(bool x) async =>
-    /*info:DOWN_CAST_COMPOSITE*/ x
-        ? 42
-        : /*@typeArgs=int*/ new MyFuture.value(42);
-Future<int> g3(bool x) async {
-  var /*@type=Object*/ y =
-      x ? 42 : /*@typeArgs=dynamic*/ new MyFuture.value(42);
-  return /*info:DOWN_CAST_COMPOSITE*/ /*@promotedType=none*/ y;
-}