Add createDirectory() to MemoryFileSystemEntity.

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';
 
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations on an
 /// in-memory virtual file system.
 ///
 /// Not intended to be implemented or extended by clients.
 class MemoryFileSystem implements FileSystem {
   final Map<Uri, Uint8List> _files = {};
+  final Set<Uri> _directories = new Set<Uri>();
 
   /// The "current directory" in the in-memory virtual file system.
   ///
   /// This is used to convert relative URIs to absolute URIs.
   ///
   /// Always ends in a trailing '/'.
   Uri currentDirectory;
 
   MemoryFileSystem(Uri currentDirectory)
       : currentDirectory = _addTrailingSlash(currentDirectory);
@@ skipping 24 matching lines @@
 
   @override
   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.
+  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 => _fileSystem._files[uri] != null;
+  Future<bool> exists() async {
+    return _fileSystem._files[uri] != null ||
+        _fileSystem._directories.contains(uri);
+  }
 
   @override
   Future<List<int>> readAsBytes() async {
     List<int> contents = _fileSystem._files[uri];
     if (contents == null) {
       throw new FileSystemException(uri, 'File $uri does not exist.');
     }
     return contents.toList();
   }
 
@@ skipping 22 matching lines @@
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsStringSync(String s) {
     // Note: the return type of UTF8.encode is List<int>, but in practice it
     // always returns Uint8List.  We rely on that for efficiency, so that we
     // don't have to make an extra copy.
     _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;
   }
 }