Rework standalone to use a synchronous loader that does not invoke Dart code

runtime/bin/builtin_impl_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 C++ sources for the dart:_builtin library and
 # some of the C++ sources for the dart:io library.  The rest are in
 # io_impl_sources.gypi.
 {
   'sources': [
     'crypto.cc',
@@ skipping 29 matching lines @@
     'file_linux.cc',
     'file_macos.cc',
     'file_support.cc',
     'file_unsupported.cc',
     'file_test.cc',
     'file_win.cc',
     'hashmap_test.cc',
     'io_buffer.cc',
     'io_buffer.h',
     'isolate_data.h',
+    'loader.cc',
+    'loader.h',
     'lockers.h',
     'thread.h',
     'thread_android.cc',
     'thread_android.h',
     'thread_linux.cc',
     'thread_linux.h',
     'thread_macos.cc',
     'thread_macos.h',
     'thread_win.cc',
     'thread_win.h',
     'utils.h',
     'utils_android.cc',
     'utils_linux.cc',
     'utils_macos.cc',
     'utils_win.cc',
     'utils_win.h',
   ],
 }