Add type checks to directory listing.

runtime/bin/directory.dart

Index: runtime/bin/directory.dart
diff --git a/runtime/bin/directory.dart b/runtime/bin/directory.dart
index 072adb68ca19afd11a5a0bc8a6d97bb40fc30611..005dd924b2097152d14d41cfc7378b93fe2da0ce 100644
--- a/runtime/bin/directory.dart
+++ b/runtime/bin/directory.dart
@@ -59,7 +59,7 @@ interface Directory factory _Directory {
   void setDoneHandler(void doneHandler(bool completed));
 
   /**
-   * Sets the error handler that is called on errors listing
+   * Sets the error handler that is called on error listing
    * directories.
    */
   void setErrorHandler(void errorHandler(String error));
@@ -69,3 +69,10 @@ interface Directory factory _Directory {
    */
   final String path;
 }
+
+
+class DirectoryException {
+  const DirectoryException(String this.message);
+  String toString() => "DirectoryException: $message";
+  final String message;
+}