Add .fromPath constructors to File and Directory.

runtime/bin/directory.dart

Index: runtime/bin/directory.dart
diff --git a/runtime/bin/directory.dart b/runtime/bin/directory.dart
index 1fd96c9f222b1ee75868625c01641f8a439d9847..60e6bfd566ba9cb86ff9d2caca6346127841f604 100644
--- a/runtime/bin/directory.dart
+++ b/runtime/bin/directory.dart
@@ -7,13 +7,20 @@
  */
 interface Directory default _Directory {
   /**
-   * Creates a directory object. The path is either a full path or
-   * relative to the directory in which the Dart VM was
-   * started.
+   * Creates a directory object. The path is either an absolute path,
+   * or it is a relative path which is interpreted relative to the directory
+   * in which the Dart VM was started.
    */
   Directory(String path);
 
   /**
+   * Creates a directory object from a Path object. The path is either
+   * an absolute path, or it is a relative path which is interpreted
+   * relative to the directory in which the Dart VM was started.
+   */
+  Directory.fromPath(Path path);
+
+  /**
    * Creates a directory object pointing to the current working
    * directory.
    */