fix #25944, improve Future.then inference

pkg/analyzer/test/src/context/mock_sdk.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.test.src.context.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
@@ skipping 11 matching lines @@
 
 part 'stream.dart';
 
 class Future<T> {
   factory Future(computation()) => null;
   factory Future.delayed(Duration duration, [T computation()]) => null;
   factory Future.value([T value]) => null;
 
   static Future<List/*<T>*/> wait/*<T>*/(
       Iterable<Future/*<T>*/> futures) => null;
-  Future/*<R>*/ then/*<R>*/(/*=R*/ onValue(T value)) => null;
+  Future/*<R>*/ then/*<R>*/(onValue(T value)) => null;
 }
 
 abstract class Completer<T> {
   factory Completer() => new _AsyncCompleter<T>();
   factory Completer.sync() => new _SyncCompleter<T>();
   Future<T> get future;
   void complete([value]);
   void completeError(Object error, [StackTrace stackTrace]);
   bool get isCompleted;
 }
@@ skipping 400 matching lines @@
 
   @override
   AnalysisCache createCacheFromSourceFactory(SourceFactory factory) {
     if (factory == null) {
       return super.createCacheFromSourceFactory(factory);
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }