Remove faulty function return type in builtin.dart

runtime/bin/builtin.dart

 // 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.
 
 library builtin;
 import 'dart:io';
 import 'dart:async';
 // import 'root_library'; happens here from C Code
 
 // The root library (aka the script) is imported into this library. The
@@ skipping 248 matching lines @@
                     _packageRoot;
   return _filePathFromUri(packageRoot.resolve(uri.path).toString());
 }
 
 
 // Returns the directory part, the filename part, and the name
 // of a native extension URL as a list [directory, filename, name].
 // The directory part is either a file system path or an HTTP(S) URL.
 // The filename part is the extension name, with the platform-dependent
 // prefixes and extensions added.
-String _extensionPathFromUri(String userUri) {
+_extensionPathFromUri(String userUri) {
   if (!userUri.startsWith(_DART_EXT)) {
     throw 'Unexpected internal error: Extension URI $userUri missing dart-ext:';
   }
   userUri = userUri.substring(_DART_EXT.length);
 
   if (userUri.contains('\\')) {
     throw 'Unexpected internal error: Extension URI $userUri contains \\';
   }
 
   String filename;
@@ skipping 19 matching lines @@
   } else if (Platform.isWindows) {
     filename = '$name.dll';
   } else {
     _logResolution(
         'Native extensions not supported on ${Platform.operatingSystem}');
     throw 'Native extensions not supported on ${Platform.operatingSystem}';
   }
 
   return [path, filename, name];
 }