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

tools/test.dart

Index: tools/test.dart
diff --git a/tools/test.dart b/tools/test.dart
index 541bdb9b0eb8021f278c36661d5d6304f353c691..0c8ddd409647a2670ad38d488522308b244d8c91 100755
--- a/tools/test.dart
+++ b/tools/test.dart
@@ -25,6 +25,7 @@
 
 #library("test");
 
+#import("dart:io");
 #import("testing/dart/test_runner.dart");
 #import("testing/dart/test_options.dart");
 #import("testing/dart/test_suite.dart");
@@ -115,10 +116,11 @@ main() {
     }
 
     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));
       }
     }