Add .fromPath constructors to File and Directory.

runtime/bin/file.dart

Index: runtime/bin/file.dart
diff --git a/runtime/bin/file.dart b/runtime/bin/file.dart
index f55bc302ad7e58cb5144a83e7acc2df0227c70bb..eacdbf39b4054cd8d37fa0ab5440552cbaf7177f 100644
--- a/runtime/bin/file.dart
+++ b/runtime/bin/file.dart
@@ -29,6 +29,11 @@ interface File default _File {
   File(String name);
 
   /**
+   * Create a File object from a Path object.  Uses Path.toNativePath().

[Mads Ager (google), 2012/06/22 07:44:36]

Remove the 'Uses Path.toNativePath().' part of the comment?

[Bill Hesse, 2012/06/22 10:35:33]

Done.

+   */
+  File.fromPath(Path path);

[Søren Gjesse, 2012/06/22 08:06:12]

Same here - should we consider making this the default File constructor?

+
+  /**
    * Check if the file exists. Does not block and returns a
    * [:Future<bool>:].
    */