fixes #25482, flatten Futures in strong mode so Future.then works

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 library analyzer.test.src.task.strong.strong_test_helper;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 78 matching lines @@
         }
         class List<E> implements Iterable<E> {
           List([int length]);
           List.filled(int length, E fill);
         }
         class Map<K, V> {
           Iterable<K> get keys {}
         }
         ''',
   'dart:async': '''
+        library dart.async;
         class Future<T> {
           Future(computation()) {}
           Future.value(T t) {}
           static Future<List/*<T>*/> wait/*<T>*/(
               Iterable<Future/*<T>*/> futures) => null;
           Future/*<R>*/ then/*<R>*/(/*=R*/ onValue(T value)) => null;
         }
         class Stream<T> {}
   ''',
   'dart:html': '''
@@ skipping 406 matching lines @@
   UriKind get uriKind => UriKind.DART_URI;
 
   bool exists() => true;
 
   Source resolveRelative(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 
   Uri resolveRelativeUri(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 }