Re-remove ad hoc Future.then inference.

pkg/front_end/testcases/inference/future_then_upwards_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(T x) {}
-  dynamic noSuchMethod(invocation);
-  MyFuture<S> then<S>(dynamic f(T x), {Function onError}) => null;
-}
-
-void main() {
-  var /*@type=MyFuture<double>*/ f =
-      foo().then(/*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3);
-  MyFuture<int> f2 = /*error:INVALID_ASSIGNMENT*/ /*@promotedType=none*/ f;
-
-  // The unnecessary cast is to illustrate that we inferred <double> for
-  // the generic type args, even though we had a return type context.
-  MyFuture<num> f3 = /*info:UNNECESSARY_CAST*/ foo().then(
-      /*@returnType=double*/ (/*@type=dynamic*/ _) => 2.3) as MyFuture<double>;
-}
-
-MyFuture foo() => new MyFuture<int>.value(1);