fixes #27586, prefer context type in generic inference

pkg/dev_compiler/tool/input_sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 194 matching lines @@
           onDone: () {
             completer.complete(_file);
           },
           onError: error,
           cancelOnError: true);
       })
       .catchError(completer.completeError);
     return completer.future;
   }
 
-  Future<File> close() =>
+  Future<RandomAccessFile> close() =>
       _openFuture.then((openedFile) => openedFile.close());
 }
 
 
 // Class for encapsulating the native implementation of files.
 class _File extends FileSystemEntity implements File {
   final String path;
 
   // Constructor for file.
   _File(this.path) {
@@ skipping 790 matching lines @@
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException("An async operation is currently pending",
                                     path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }