Allow DirectoryDescriptor.load to load a file next to a directory.

This mkaes it easier to model servers using descriptors.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//15682008

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@23607 260f80e4-7a28-3924-810f-c04153c831b5

test/descriptor/tar.dart

@@ -15,7 +15,8 @@ import '../../lib/src/io.dart';
 import '../../lib/src/utils.dart';
 
 /// Describes a tar file and its contents.
-class TarFileDescriptor extends DirectoryDescriptor {
+class TarFileDescriptor extends DirectoryDescriptor
+    implements ReadableDescriptor {
   TarFileDescriptor(String name, List<Descriptor> contents)
       : super(name, contents);