Add ThreadBarrier; use in GCMarker and unit test

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 428 matching lines @@
     'stub_code_mips.cc',
     'stub_code_mips_test.cc',
     'stub_code_x64.cc',
     'stub_code_x64_test.cc',
     'symbols.cc',
     'symbols.h',
     'tags.cc',
     'tags.h',
     'thread.cc',
     'thread.h',
+    'thread_barrier.h',
+    'thread_barrier_test.cc',
     'thread_interrupter.cc',
     'thread_interrupter.h',
     'thread_interrupter_android.cc',
     'thread_interrupter_linux.cc',
     'thread_interrupter_macos.cc',
     'thread_interrupter_test.cc',
     'thread_interrupter_win.cc',
     'thread_pool.cc',
     'thread_pool.h',
     'thread_pool_test.cc',
@@ skipping 39 matching lines @@
     'vtune.h',
     'weak_code.cc',
     'weak_code.h',
     'weak_table.cc',
     'weak_table.h',
     'zone.cc',
     'zone.h',
     'zone_test.cc',
   ],
 }