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

tools/test-runtime.dart

 #!/usr/bin/env 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.
 
 // TODO(ager): Get rid of this version of test.dart when we don't have
 // to worry about the special runtime checkout anymore.
 // This file is identical to test.dart with test suites in the
 // directories samples, client, compiler, frog, and utils removed.
 
 #library("test");
 
+#import("dart:io");
 #import("testing/dart/test_runner.dart");
 #import("testing/dart/test_options.dart");
 #import("testing/dart/test_suite.dart");
 
 #import("../tests/co19/test_config.dart");
 #import("../runtime/tests/vm/test_config.dart");
 
 /**
  * The directories that contain test suites which follow the conventions
  * required by [StandardTestSuite]'s forDirectory constructor.
@@ skipping 51 matching lines @@
     if (selectors.containsKey('co19')) {
       queue.addTestSuite(new Co19TestSuite(conf));
     }
     if (conf['runtime'] == 'vm' && selectors.containsKey('vm')) {
       // vm tests contain both cc tests (added here) and dart tests (added in
       // [TEST_SUITE_DIRECTORIES]).
       queue.addTestSuite(new VMTestSuite(conf));
     }
 
     for (final testSuiteDir in TEST_SUITE_DIRECTORIES) {
-      final name = testSuiteDir.substring(testSuiteDir.lastIndexOf('/') + 1);
+      Path testSuitePath = new Path(testSuiteDir);
+      final name = testSuitePath.filename;
       if (selectors.containsKey(name)) {
         queue.addTestSuite(
-            new StandardTestSuite.forDirectory(conf, testSuiteDir));
+            new StandardTestSuite.forDirectory(conf, testSuitePath));
       }
     }
 
     return true;
   }
 
   // Start process queue.
   var queue = new ProcessQueue(maxProcesses,
                                progressIndicator,
                                startTime,
                                printTiming,
                                enqueueConfiguration,
                                verbose,
                                listTests);
 }