Change test scripts to use Path library in most places, instead of strings.

tests/co19/test_config.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("co19_test_config");
 
+#import("dart:io");
 #import("../../tools/testing/dart/test_suite.dart");
 
 class Co19TestSuite extends StandardTestSuite {
   RegExp _testRegExp = const RegExp(@"t[0-9]{2}.dart$");
 
   Co19TestSuite(Map configuration)
       : super(configuration,
               "co19",
-              "tests/co19/src",
+              const Path("tests/co19/src"),
               ["tests/co19/co19-compiler.status",
                "tests/co19/co19-runtime.status",
                "tests/co19/co19-frog.status",
                "tests/co19/co19-leg.status"]);
 
   bool isTestFile(String filename) => _testRegExp.hasMatch(filename);
   bool listRecursively() => true;
 }