Properly flatten FutureOr types when inferring the type of an async method.

pkg/front_end/testcases/inference/future_then_conditional_6.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>(FutureOr<S> f(T x), {Function onError}) => null;
 }
 
-void main() {
+void test() {
   Future<bool> f;
   Future<int> t1 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
       /*@returnType=Future<int>*/ (/*@type=bool*/ x) async =>
           x ? 2 : await new Future<int>.value(3));
   Future<int> t2 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
       /*@returnType=Future<int>*/ (/*@type=bool*/ x) async {
     return /*info:DOWN_CAST_COMPOSITE*/ await x ? 2 : new Future<int>.value(3);
   });
   Future<int> t5 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
       /*info:INFERRED_TYPE_CLOSURE,error:INVALID_CAST_FUNCTION_EXPR*/
       /*@returnType=Object*/ (/*@type=bool*/ x) =>
           x ? 2 : new Future<int>.value(3));
   Future<int> t6 = f. /*@typeArgs=int*/ /*@target=Future::then*/ then(
       /*@returnType=FutureOr<int>*/ (/*@type=bool*/ x) {
     return /*info:DOWN_CAST_COMPOSITE*/ x ? 2 : new Future<int>.value(3);
   });
 }
+
+main() {}