Handle FutureOr type when inferring async closures.

pkg/front_end/testcases/inference/async_closure_return_type_future_or.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';
 
 FutureOr<int> futureOrInt = null;
 var /*@topType=() -> FutureOr<int>*/ f = /*@returnType=FutureOr<int>*/ () =>
     futureOrInt;
 var /*@topType=() -> Future<int>*/ g = /*@returnType=Future<int>*/ () async =>
     futureOrInt;
+
+main() {
+  f;
+  g;
+}