Enable "part of" for the async library.

runtime/vm/vm.gypi

 # 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.
 
 {
   'variables': {
     'builtin_in_cc_file': '../bin/builtin_in.cc',
     'async_cc_file': '<(SHARED_INTERMEDIATE_DIR)/async_gen.cc',
     'async_patch_cc_file': '<(SHARED_INTERMEDIATE_DIR)/async_patch_gen.cc',
     'corelib_cc_file': '<(SHARED_INTERMEDIATE_DIR)/corelib_gen.cc',
@@ skipping 177 matching lines @@
           'outputs': [
             '<(async_cc_file)',
           ],
           'action': [
             'python',
             'tools/create_string_literal.py',
             '--output', '<(async_cc_file)',
             '--input_cc', '<(builtin_in_cc_file)',
             '--include', 'vm/bootstrap.h',
             '--var_name', 'dart::Bootstrap::async_source_',
-            '<@(_sources)',
+            '<@(async_dart)',
           ],
           'message': 'Generating ''<(async_cc_file)'' file.'
         },
       ]
     },
     {
       'target_name': 'generate_corelib_cc_file',
       'type': 'none',
       'variables': {
         'core_dart': '<(SHARED_INTERMEDIATE_DIR)/core_gen.dart',
@@ skipping 583 matching lines @@
             '--include', 'INTENTIONALLY_LEFT_BLANK',
             '--var_name', 'INTENTIONALLY_LEFT_BLANK_TOO',
             '<(snapshot_test_dart_file)',
           ],
           'message': 'Generating ''<(snapshot_test_dat_file)'' file.'
         },
       ]
     },
   ]
 }