Add createDirectory() to MemoryFileSystemEntity.

pkg/front_end/test/memory_file_system_test.dart

Index: pkg/front_end/test/memory_file_system_test.dart
diff --git a/pkg/front_end/test/memory_file_system_test.dart b/pkg/front_end/test/memory_file_system_test.dart
index f0b44533d9816c1e308ea980ad39ed91a390e36f..986abb70fe139a55df95d8e5543ea97d138ff60d 100644
--- a/pkg/front_end/test/memory_file_system_test.dart
+++ b/pkg/front_end/test/memory_file_system_test.dart
@@ -37,6 +37,21 @@ class FileTest extends _BaseTestNative {
     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);
   }
@@ -45,11 +60,16 @@ class FileTest extends _BaseTestNative {
     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);
   }
@@ -99,6 +119,11 @@ class FileTest extends _BaseTestNative {
     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;
@@ -118,6 +143,11 @@ class FileTest extends _BaseTestNative {
     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');