Set up a variable so that the compiler script can be imported using an import map.

bin/file_win.cc

 // 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.
 
 #include "bin/file.h"
 
 #include <fcntl.h>
 #include <io.h>
 #include <stdio.h>
 #include <string.h>
@@ skipping 197 matching lines @@
   file_part[-1] = '\0';
   return path;
 }
 
 
 const char* File::PathSeparator() {
   return "\\";
 }
 
 
+char File::PathSeparatorCharacter() {
+  return '\\';
+}
+
+
 const char* File::StringEscapedPathSeparator() {
   return "\\\\";
 }
 
 
 File::StdioHandleType File::GetStdioHandleType(int fd) {
   // Treat all stdio handles as pipes. The Windows event handler and
   // socket code will handle the different handle types.
   return kPipe;
 }