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

tests/standalone/io/process_shell_test.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 "package:path/path.dart";
 import "package:async_helper/async_helper.dart";
 import "dart:io";
 import "dart:async";
 import "dart:isolate";
 
 void testRunShell() {
   test(args) {
     asyncStart();
-    var path = Platform.script.resolve("process_echo_util.dart").toFilePath();
+    var path = join(dirname(Platform.script), "process_echo_util.dart");
     Process.run(Platform.executable,
                 [path]..addAll(args),
                 runInShell: true)
         .then((result) {
           if (Platform.operatingSystem == "windows") {
             result = result.stdout.split("\r\n");
           } else {
             result = result.stdout.split("\n");
           }
           if (result.length - 1 != args.length) {
@@ skipping 33 matching lines @@
   }
   test("'\"'");
   test("'\$HOME'");
 }
 
 void main() {
   testRunShell();
   testBadRunShell();
 }