Fix dartbug.com/3424:

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:uri");
 
 // Corelib 'print' implementation.
 void _print(arg) {
   _Logger._printString(arg.toString());
@@ skipping 12 matching lines @@
 // The directory to look in to resolve "package:" scheme URIs.
 var _packageRoot;
 
 void _logResolution(String msg) {
   final enabled = false;
   if (enabled) {
     _Logger._printString(msg);
   }
 }
 
-String _setPackageRoot(String packageRoot) {
+_setPackageRoot(String packageRoot) {
   // TODO(mattsh) - refactor windows drive and path handling code
   // so it can be used here if needed.
   _packageRoot = packageRoot;
 }
 
 String _resolveScriptUri(String cwd, String scriptName, bool isWindows) {
   _logResolution("# Current working directory: $cwd");
   _logResolution("# ScriptName: $scriptName");
   if (isWindows) {
     // For Windows we need to massage the paths a bit according to
@@ skipping 105 matching lines @@
   var path;
   if (_packageRoot !== null) {
     path = "${_packageRoot}${uri.path}";
   } else {
     path = _entrypoint.resolve('packages/${uri.path}').path;
   }
 
   _logResolution("# Package: $path");
   return path;
 }