Change dart:io Platform.script to return a Uri.

tests/standalone/io/regress_7191_script.dart

 // 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.
 
 import 'dart:io';
 import 'dart:isolate';
 
 main() {
   // Open a port to make the script hang.
   var port = new ReceivePort();
   // Start sub-process when receiving data.
   var subscription;
   subscription = stdin.listen((data) {
-    Process.start(Platform.executable, [Platform.script]).then((p) {
+    Process.start(Platform.executable,
+                  [Platform.script.toFilePath()]).then((p) {
       p.stdout.listen((_) { });
       p.stderr.listen((_) { });
       // When receiving data again, kill sub-process and exit.
       subscription.onData((data) {
         // If a SIGTERM is sent before the child-process's main is invoked,
         // there is a change that the SIGTERM is ignore on Mac OS X. Use
         // SIGKILL to get around the issue.
         p.kill(ProcessSignal.SIGKILL);
         p.exitCode.then((_) => exit(0));
       });
       // Close stdout. If handles are incorrectly inherited this will
       // not actually close stdout and the test will hang.
       stdout.close();
     });
   });
 }