- Add missing header file to vm_sources.gypi.

runtime/vm/vm_sources.gypi

 # 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.
 
 # This file contains all sources (vm and tests) for the dart virtual machine.
 # Unit test files need to have a '_test' suffix appended to the name.
 {
   'sources': [
     'allocation.cc',
     'allocation.h',
@@ skipping 236 matching lines @@
     'native_symbol_macos.cc',
     'native_symbol_win.cc',
     'object.cc',
     'object.h',
     'object_arm_test.cc',
     'object_ia32_test.cc',
     'object_id_ring.cc',
     'object_id_ring.h',
     'object_id_ring_test.cc',
     'object_mips_test.cc',
+    'object_set.h',
     'object_store.cc',
     'object_store.h',
     'object_store_test.cc',
     'object_test.cc',
     'object_x64_test.cc',
     'os.h',
     'os_android.cc',
     'os_linux.cc',
     'os_macos.cc',
     'os_test.cc',
@@ skipping 105 matching lines @@
     'visitor.h',
     'vtune.cc',
     'vtune.h',
     'weak_table.cc',
     'weak_table.h',
     'zone.cc',
     'zone.h',
     'zone_test.cc',
   ],
 }