Add createDirectory() to MemoryFileSystemEntity.

pkg/front_end/test/memory_file_system_test.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.
 // SharedOptions=--supermixin
 
 library front_end.test.memory_file_system_test;
 
 import 'dart:convert';
 import 'dart:io' as io;
 
@@ skipping 19 matching lines @@
 class FileTest extends _BaseTestNative {
   String path;
   MemoryFileSystemEntity file;
 
   setUp() {
     super.setUp();
     path = join(tempPath, 'file.txt');
     file = entityForPath(path);
   }
 
+  test_createDirectory_doesNotExist() async {
+    file.createDirectory();
+    expect(await file.exists(), true);
+  }
+
+  test_createDirectory_exists_asDirectory() async {
+    file.createDirectory();
+    expect(() => file.createDirectory(), _throwsFileSystemException);
+  }
+
+  test_createDirectory_exists_asFile() async {
+    file.writeAsStringSync('');
+    expect(() => file.createDirectory(), _throwsFileSystemException);
+  }
+
   test_equals_differentPaths() {
     expect(file == entityForPath(join(tempPath, 'file2.txt')), isFalse);
   }
 
   test_equals_samePath() {
     expect(file == entityForPath(join(tempPath, 'file.txt')), isTrue);
   }
 
+  test_exists_directory_exists() async {
+    file.createDirectory();
+    expect(await file.exists(), true);
+  }
+
   test_exists_doesNotExist() async {
     expect(await file.exists(), false);
   }
 
-  test_exists_exists() async {
+  test_exists_file_exists() async {
     file.writeAsStringSync('x');
     expect(await file.exists(), true);
   }
 
   test_hashCode_samePath() {
     expect(file.hashCode, entityForPath(join(tempPath, 'file.txt')).hashCode);
   }
 
   test_path() {
     expect(file.uri, context.toUri(path));
@@ skipping 29 matching lines @@
     var s = 'contents';
     file.writeAsStringSync(s);
     expect(await file.readAsString(), s);
   }
 
   test_readAsString_utf8() async {
     file.writeAsBytesSync([0xe2, 0x82, 0xac]); // Unicode € symbol, in UTF-8
     expect(await file.readAsString(), '\u20ac');
   }
 
+  test_writeAsBytesSync_directory() async {
+    file.createDirectory();
+    expect(() => file.writeAsBytesSync([0]), _throwsFileSystemException);
+  }
+
   test_writeAsBytesSync_modifyAfterRead() async {
     file.writeAsBytesSync([1]);
     (await file.readAsBytes())[0] = 2;
     expect(await file.readAsBytes(), [1]);
   }
 
   test_writeAsBytesSync_modifyAfterWrite() async {
     var bytes = [1];
     file.writeAsBytesSync(bytes);
     bytes[0] = 2;
     expect(await file.readAsBytes(), [1]);
   }
 
   test_writeAsBytesSync_overwrite() async {
     file.writeAsBytesSync([1]);
     file.writeAsBytesSync([2]);
     expect(await file.readAsBytes(), [2]);
   }
 
+  test_writeAsStringSync_directory() async {
+    file.createDirectory();
+    expect(() => file.writeAsStringSync(''), _throwsFileSystemException);
+  }
+
   test_writeAsStringSync_overwrite() async {
     file.writeAsStringSync('first');
     file.writeAsStringSync('second');
     expect(await file.readAsString(), 'second');
   }
 
   test_writeAsStringSync_utf8() async {
     file.writeAsStringSync('\u20ac'); // Unicode € symbol
     expect(await file.readAsBytes(), [0xe2, 0x82, 0xac]);
   }
@@ skipping 138 matching lines @@
   String tempPath;
 
   String join(String path1, String path2, [String path3, String path4]) =>
       pathos.windows.join(path1, path2, path3, path4);
 
   void setUp() {
     tempPath = r'c:\test_file_system';
     fileSystem = new MemoryFileSystem(Uri.parse('file:///c:/cwd'));
   }
 }