Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_then_conditional_deprecated.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(T x) {}
-  dynamic noSuchMethod(invocation);
-  MyFuture<S> then<S>(dynamic f(T x), {Function onError}) => null;
-}
-
-void main() {
-  MyFuture<bool> f;
-  Future<int> t1 = /*@promotedType=none*/ f.then(
-      /*@returnType=Future<int>*/ (/*@type=bool*/ x) async =>
-          x ? 2 : await new Future<int>.value(3));
-  Future<int> t2 = /*@promotedType=none*/ f
-      .then(/*@returnType=Future<int>*/ (/*@type=bool*/ x) async {
-    return /*info:DOWN_CAST_COMPOSITE*/ await x ? 2 : new Future<int>.value(3);
-  });
-  Future<int> t5 = /*@promotedType=none*/ f.then(
-      /*@returnType=Object*/ (/*@type=bool*/ x) =>
-          x ? 2 : new Future<int>.value(3));
-  Future<int> t6 = /*@promotedType=none*/ f.then(
-      /*@returnType=FutureOr<int>*/ (/*@type=bool*/ x) {
-    return /*info:DOWN_CAST_COMPOSITE*/ x ? 2 : new Future<int>.value(3);
-  });
-}