Implement spawnUri from dart:isolate. This function allows us to

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");
 
 void print(arg) {
   _Logger._printString(arg.toString());
 }
 
 void exit(int status) {
   if (status is !int) {
     throw new IllegalArgumentException("int status expected");
   }
   _exit(status);
 }
 
 _exit(int status) native "Exit";
 
 class _Logger {
   static void _printString(String s) native "Logger_PrintString";
 }
 
 
-// Code to deal with URI resolution for the standalone binary.
-// For Windows we need to massage the paths a bit according to
-// http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
-var _is_windows;
-
 // The URI that the entrypoint script was loaded from. Remembered so that
 // package imports can be resolved relative to it.
 var _entrypoint;
 
 // 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) {
   // 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 windows) {
-  _is_windows = windows;
+String _resolveScriptUri(String cwd, String scriptName, bool isWindows) {
   _logResolution("# Current working directory: $cwd");
   _logResolution("# ScriptName: $scriptName");
-  if (windows) {
+  if (isWindows) {
+    // For Windows we need to massage the paths a bit according to
+    // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
+    //
     // Convert
     // C:\one\two\three
     // to
     // /C:/one/two/three
     cwd = "/${cwd.replaceAll('\\', '/')}";
     _logResolution("## cwd: $cwd");
     if ((scriptName.length > 2) && (scriptName[1] == ":")) {
       // This is an absolute path.
       scriptName = "/${scriptName.replaceAll('\\', '/')}";
     } else {
@@ skipping 19 matching lines @@
     resolved = baseUri.resolve(uri.path);
     resolved = new Uri(scheme: "dart-ext", path: resolved.path);
   } else {
     resolved = baseUri.resolve(userString);
   }
   _logResolution("# Resolved to: $resolved");
   return resolved.toString();
 }
 
 
-String _filePathFromUri(String userUri) {
+String _filePathFromUri(String userUri, bool isWindows) {
   var uri = new Uri.fromString(userUri);
   _logResolution("# Getting file path from: $uri");
 
   var path;
   switch (uri.scheme) {
     case 'file':
       path = _filePathFromFileUri(uri);
       break;
     case 'dart-ext':
       path = _filePathFromOtherUri(uri);
       break;
     case 'package':
       path = _filePathFromPackageUri(uri);
       break;
     default:
       // Only handling file and package URIs in standalone binary.
       _logResolution("# Unknown scheme (${uri.scheme}) in $uri.");
       throw "Not a known scheme: $uri";
   }
 
-  if (_is_windows) {
+  if (isWindows) {
+    // For Windows we need to massage the paths a bit according to
+    // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
+    //
     // Drop the leading / before the drive letter.
     path = path.substring(1);
     _logResolution("# path: $path");
   }
 
   return path;
 }
 
 String _filePathFromFileUri(Uri uri) {
   if (uri.domain != '') {
@@ skipping 26 matching lines @@
   var path;
   if (_packageRoot !== null) {
     path = "${_packageRoot}${uri.path}";
   } else {
     path = _entrypoint.resolve('packages/${uri.path}').path;
   }
 
   _logResolution("# Package: $path");
   return path;
 }