diff --git a/pkg/front_end/lib/memory_file_system.dart b/pkg/front_end/lib/memory_file_system.dart
index b5515b77251..01513496867 100644
--- a/pkg/front_end/lib/memory_file_system.dart
+++ b/pkg/front_end/lib/memory_file_system.dart
@@ -16,6 +16,7 @@ import 'file_system.dart';
 /// 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.
   ///
@@ -60,8 +61,25 @@ class MemoryFileSystemEntity implements FileSystemEntity {
       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 {
@@ -104,6 +122,9 @@ class MemoryFileSystemEntity implements FileSystemEntity {
   }
 
   void _update(Uri uri, Uint8List data) {
+    if (_fileSystem._directories.contains(uri)) {
+      throw new FileSystemException(uri, 'Entry $uri is a directory.');
+    }
     _fileSystem._files[uri] = data;
   }
 }
diff --git a/pkg/front_end/test/memory_file_system_test.dart b/pkg/front_end/test/memory_file_system_test.dart
index f0b44533d98..986abb70fe1 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');