Remove uses of Path class from tests/standalone.

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 "dart:io";
 import "dart:isolate";
 
 void testRunShell() {
   test(args) {
-    var path = new Path(Platform.script);
-    path = path.directoryPath.join(new Path("process_echo_util.dart"));
+    var path = join(dirname(Platform.script), "process_echo_util.dart");
     Process.run(Platform.executable,
-                [path.toString()]..addAll(args),
+                [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) {
             throw "wrong number of args: $args vs $result";
           }
@@ skipping 12 matching lines @@
   test(["'\"\"'", "\"'\"'"]);
   test(["'\\\"\\\"'\\", "\"\\'\"'"]);
   test(["'\$HOME'"]);
   test(["'\$tmp'"]);
   test(["arg'"]);
   test(["arg\\'", "'\\arg"]);
 }
 
 void testBadRunShell() {
   test(exe, [args = const []]) {
-    var path = new Path(Platform.script);
-    path = path.directoryPath.join(new Path("process_echo_util.dart"));
     Process.run(exe, args, runInShell: true)
         .then((result) {
           port.close();
           if (result.exitCode == 0) {
             throw "error expected";
           }
         });
   }
   test("'\"'");
   test("'\$HOME'");
 }
 
 void main() {
   testRunShell();
   testBadRunShell();
 }