Speed up freelist by using intrinsics to find last set bit.

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 29 matching lines @@
     'atomic_android.h',
     'atomic_linux.h',
     'atomic_macos.h',
     'atomic_win.h',
     'base_isolate.h',
     'benchmark_test.cc',
     'benchmark_test.h',
     'bigint_operations.cc',
     'bigint_operations.h',
     'bigint_operations_test.cc',
+    'bit_set_test.cc',
     'bit_vector.cc',
     'bit_vector.h',
     'bit_vector_test.cc',
     'bitfield.h',
     'bitfield_test.cc',
     'bitmap.cc',
     'bitmap.h',
     'bitmap_test.cc',
     'block_scheduler.cc',
     'block_scheduler.h',
@@ skipping 388 matching lines @@
     'visitor.h',
     'vtune.cc',
     'vtune.h',
     'weak_table.cc',
     'weak_table.h',
     'zone.cc',
     'zone.h',
     'zone_test.cc',
   ],
 }