Regularize some naming to make BUILD.gn files easier to write.

runtime/bin/bin.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.
 
 {
   'variables': {
     'gen_source_dir': '<(SHARED_INTERMEDIATE_DIR)',
 
     'io_cc_file': '<(gen_source_dir)/io_gen.cc',
     'io_patch_cc_file': '<(gen_source_dir)/io_patch_gen.cc',
@@ skipping 22 matching lines @@
           ],
           'outputs': [
             '<(builtin_cc_file)',
           ],
           'action': [
             'python',
             'tools/gen_library_src_paths.py',
             '--output', '<(builtin_cc_file)',
             '--input_cc', '<(builtin_in_cc_file)',
             '--include', 'bin/builtin.h',
-            '--var_name', 'dart::bin::Builtin::builtin_source_paths_',
+            '--var_name', 'dart::bin::Builtin::_builtin_source_paths_',
             '--library_name', 'dart:_builtin',
             '<@(_sources)',
           ],
           'message': 'Generating ''<(builtin_cc_file)'' file.'
         },
       ]
     },
     {
       'target_name': 'generate_io_cc_file',
       'type': 'none',
@@ skipping 536 matching lines @@
         }],
         ['OS=="linux"', {
           'cflags': [
             '-fPIC',
           ],
         }],
       ],
     },
   ],
 }