Add support for SDK outline in KernelDriver.

pkg/front_end/lib/memory_file_system.dart

 // Copyright (c) 2016, 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 front_end.memory_file_system;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
@@ skipping 45 matching lines @@
   int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is MemoryFileSystemEntity &&
       other.uri == uri &&
       identical(other._fileSystem, _fileSystem);
 
   /// Create a directory for this file system entry.
   ///
-  /// If the entry already exists, either as a file, or as a directory,
-  /// this is an error.
+  /// If the entry is an existing file, this is an error.
   void createDirectory() {
     if (_fileSystem._files[uri] != null) {
       throw new FileSystemException(uri, 'Entry $uri is a file.');
     }
-    if (_fileSystem._directories.contains(uri)) {
-      throw new FileSystemException(uri, 'Directory $uri already exists.');
-    }
     _fileSystem._directories.add(uri);
   }
 
   @override
   Future<bool> exists() async {
     return _fileSystem._files[uri] != null ||
         _fileSystem._directories.contains(uri);
   }
 
   @override
@@ skipping 36 matching lines @@
     _update(uri, UTF8.encode(s) as Uint8List);
   }
 
   void _update(Uri uri, Uint8List data) {
     if (_fileSystem._directories.contains(uri)) {
       throw new FileSystemException(uri, 'Entry $uri is a directory.');
     }
     _fileSystem._files[uri] = data;
   }
 }