Implement toString on File and Directory objects.

sdk/lib/io/directory.dart

Index: sdk/lib/io/directory.dart
diff --git a/sdk/lib/io/directory.dart b/sdk/lib/io/directory.dart
index bdc73332dfb7c08687de21da42c0e51471a5da5c..42411ac712801b1d398e37009488cf69667dab92 100644
--- a/sdk/lib/io/directory.dart
+++ b/sdk/lib/io/directory.dart
@@ -140,6 +140,11 @@ abstract class Directory {
   DirectoryLister list({bool recursive: false});
 
   /**
+   * Returns a human readable string for this Directory instance.
+   */
+  String toString();
+
+  /**
    * Gets the path of this directory.
    */
   final String path;