Change Directory.create to not fail on existing directory with that name.

runtime/bin/directory.dart

Index: runtime/bin/directory.dart
diff --git a/runtime/bin/directory.dart b/runtime/bin/directory.dart
index d7fab763dfa9e1538214d586157bc5bfb8547040..c4b6b5948cb9099441540b38da27ccd2e6a35617 100644
--- a/runtime/bin/directory.dart
+++ b/runtime/bin/directory.dart
@@ -38,15 +38,18 @@ abstract class Directory {
   bool existsSync();
 
   /**
-   * Creates the directory with this name if it does not
-   * exist. Returns a [:Future<Directory>:] that completes with this
-   * directory once it has been created.
+   * Creates the directory with this name. If the directory already
+   * exists nothing is done. Returns a [:Future<Directory>:] that
+   * completes with this directory once it has been created. If the
+   * directory does not exist and cannot be created the future
+   * completes with an exception.
    */
   Future<Directory> create();
 
   /**
-   * Synchronously creates the directory with this name if it does not exist.
-   * Throws an exception if the directory already exists.
+   * Synchronously creates the directory with this name. If the
+   * directory already exists nothing is done. If the directory does
+   * not exist and cannot be created an exception is thrown.
    */
   void createSync();