Make Source.getDirectory synchronous.

lib/src/source/unknown.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.source.unknown;
 
 import 'dart:async';
 
 import '../package.dart';
 import '../pubspec.dart';
@@ skipping 17 matching lines @@
 
   int get hashCode => name.hashCode;
 
   Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
       "Cannot describe a package from unknown source '$name'.");
 
   Future get(PackageId id, String symlink) => throw new UnsupportedError(
       "Cannot get an unknown source '$name'.");
 
   /// Returns the directory where this package can be found locally.
-  Future<String> getDirectory(PackageId id) => throw new UnsupportedError(
+  String getDirectory(PackageId id) => throw new UnsupportedError(
       "Cannot find a package from an unknown source '$name'.");
 
   bool descriptionsEqual(description1, description2) =>
       description1 == description2;
 
   /// Unknown sources do no validation.
   dynamic parseDescription(String containingPath, description,
                            {bool fromLockFile: false}) => description;
 }