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

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
===================================================================
--- runtime/bin/builtin.dart	(revision 7714)
+++ runtime/bin/builtin.dart	(working copy)
@@ -23,11 +23,6 @@
 }
 
 
-// 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;
@@ -48,11 +43,13 @@
   _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
@@ -92,7 +89,7 @@
 }
 
 
-String _filePathFromUri(String userUri) {
+String _filePathFromUri(String userUri, bool isWindows) {
   var uri = new Uri.fromString(userUri);
   _logResolution("# Getting file path from: $uri");
 
@@ -113,7 +110,10 @@
       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");