Implement File.directory() and File.directorySync() to get

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index ed86895d24a95f229ebbbc07ee1a5e0e2b2d285b..62510de21c38919972f8cc10663979ccbc2e825d 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -320,6 +320,21 @@ void FUNCTION_NAME(File_Delete)(Dart_NativeArguments args) {
 }
 
 
+void FUNCTION_NAME(File_Directory)(Dart_NativeArguments args) {
+  Dart_EnterScope();
+  const char* str =
+      DartUtils::GetStringValue(Dart_GetNativeArgument(args, 0));
+  char* str_copy = strdup(str);
+  char* path = File::GetContainingDirectory(str_copy);
+  if (path != NULL) {
+    Dart_SetReturnValue(args, Dart_NewString(path));
+  }
+  free(str_copy);
+  free(path);
+  Dart_ExitScope();
+}
+
+
 void FUNCTION_NAME(File_FullPath)(Dart_NativeArguments args) {
   Dart_EnterScope();
   const char* str =