Work around a Gyp bug by creating a stamp file for the http package.

pkg/pkg_files.gyp

Index: pkg/pkg_files.gyp
diff --git a/pkg/pkg_files.gyp b/pkg/pkg_files.gyp
index 787e5dd3aeb11cf9032ca64d093c28b27098beff..e091dcd9377ee29ec6c13825798f26a96f47d1bb 100644
--- a/pkg/pkg_files.gyp
+++ b/pkg/pkg_files.gyp
@@ -43,6 +43,26 @@
           ],
         },
       ],
-    }
+    },
+    {
+      'target_name': 'http_files_stamp',
+      'type': 'none',
+      'actions': [
+        {
+          'action_name': 'make_http_files_stamp',
+          'inputs': [
+            '../tools/create_timestamp_file.py',
+            '<!@(["python", "../tools/list_files.py", "\\.dart$", "http/lib"])',
+          ],
+          'outputs': [
+            '<(SHARED_INTERMEDIATE_DIR)/http_files.stamp',
+          ],
+          'action': [
+            'python', '../tools/create_timestamp_file.py',
+            '<@(_outputs)',
+          ],
+        },
+      ],
+    },
   ],
 }