Eliminate unused url mapping parameter in dartutils

runtime/bin/gen_snapshot.cc

 // 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.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 333 matching lines @@
     }
   }
 
   // Get the file path out of the url.
   Dart_Handle file_path = FilePathFromUri(
       DartUtils::GetStringValue(resolved_url));
   if (Dart_IsError(file_path)) {
     return file_path;
   }
 
-  return DartUtils::LoadSource(NULL,
-                               library,
+  return DartUtils::LoadSource(library,
                                url,
                                tag,
                                DartUtils::GetStringValue(file_path));
 }
 
 
 static Dart_Handle LoadSnapshotCreationScript(const char* script_name) {
   Dart_Handle resolved_script_uri = ResolveScriptUri(script_name);
   if (Dart_IsError(resolved_script_uri)) {
     return resolved_script_uri;
@@ skipping 201 matching lines @@
   }
   return 0;
 }
 
 }  // namespace bin
 }  // namespace dart
 
 int main(int argc, char** argv) {
   return dart::bin::main(argc, argv);
 }