Add packages as a dependency for dart2js target

pkg/pkg.gyp

Index: pkg/pkg.gyp
===================================================================
--- pkg/pkg.gyp	(revision 23664)
+++ pkg/pkg.gyp	(working copy)
@@ -23,6 +23,20 @@
             '<@(_inputs)',
           ],
         },
+        {
+          'action_name': 'make_pkg_timestamp',
+          'inputs': [
+            '../tools/create_timestamp_file.py',
+            '<!@(["python", "../tools/list_pkg_directories.py", "."])',
+          ],
+          'outputs': [
+            '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
+          ],
+          'action': [
+            'python', '../tools/create_timestamp_file.py',
+            '<(SHARED_INTERMEDIATE_DIR)/packages.stamp',
+          ],
+        },
       ],
     }
   ],