Add support for SDK outline in KernelDriver.

pkg/front_end/lib/memory_file_system.dart

Index: pkg/front_end/lib/memory_file_system.dart
diff --git a/pkg/front_end/lib/memory_file_system.dart b/pkg/front_end/lib/memory_file_system.dart
index 015134968670bcb8985020c19a478adee1e9d0f9..ea9a1dfcaa9b70b91f564fa7ef2034e8e9a3eb16 100644
--- a/pkg/front_end/lib/memory_file_system.dart
+++ b/pkg/front_end/lib/memory_file_system.dart
@@ -63,15 +63,11 @@ class MemoryFileSystemEntity implements FileSystemEntity {
 
   /// 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);
   }