Change the thread interface in runtime/platform and use it starting all threads

runtime/bin/builtin_sources.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.
 
 # 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': [
     #
     # Dart sources.
@@ skipping 58 matching lines @@
     'process_win.cc',
     'socket.cc',
     'socket.h',
     'socket_linux.cc',
     'socket_macos.cc',
     'socket_win.cc',
     'set.h',
     'set_test.cc',
     'thread.h',
     'thread_pool.cc',
-    'thread_pool_linux.cc',
-    'thread_pool_linux.h',
-    'thread_pool_macos.cc',
-    'thread_pool_macos.h',
     'thread_pool_test.cc',
-    'thread_pool_win.cc',
-    'thread_pool_win.h',
   ],
 }