Allow tests to specify a package root.

dart/dart.gyp

 # Copyright (c) 2012, 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.
 
 {
   'targets': [
     {
       'target_name': 'compiler',
       'type': 'none',
       'dependencies': [
@@ skipping 107 matching lines @@
         'utils/apidoc/apidoc.gyp:api_docs',
       ],
     },
     {
       'target_name': 'samples',
       'type': 'none',
       'dependencies': [
         'samples/sample_extension/sample_extension.gyp:sample_extension',
       ],
     },
+    {
+      'target_name': 'packages',
+      'type': 'none',
+      'dependencies': [
+        'pkg/pkg.gyp:pkg_packages',
+      ],
+    },
   ],
 }