Add explicit package spec to snapshot build steps

utils/pub/pub.gyp

 # Copyright (c) 2013, 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': 'pub',
       'type': 'none',
       'dependencies': [
         '../../runtime/dart-runtime.gyp:dart',
         '../../pkg/pkg_files.gyp:pkg_files_stamp',
         '../../utils/compiler/compiler.gyp:dart2js_files_stamp'
       ],
       'actions': [
         {
           'action_name': 'generate_pub_snapshot',
           'inputs': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
             '../../sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart',
             '<(SHARED_INTERMEDIATE_DIR)/dart2js_files.stamp',
             '<(SHARED_INTERMEDIATE_DIR)/pkg_files.stamp',
           ],
           'outputs': [
             '<(SHARED_INTERMEDIATE_DIR)/pub.dart.snapshot',
           ],
           'action': [
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)dart<(EXECUTABLE_SUFFIX)',
+            '--packages=../../.packages',
             '--snapshot=<(SHARED_INTERMEDIATE_DIR)/pub.dart.snapshot',
             '../../third_party/pkg/pub/bin/pub.dart',
           ]
         },
       ],
     },
   ],
 }