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

tools/test-runtime.dart

Index: tools/test-runtime.dart
diff --git a/tools/test-runtime.dart b/tools/test-runtime.dart
index d3471a68cb05e19f6e0a2b301485f739067e8715..82e12c2489c28a23bcb7d0eaf077c8303dc53393 100755
--- a/tools/test-runtime.dart
+++ b/tools/test-runtime.dart
@@ -10,6 +10,7 @@
 
 #library("test");
 
+#import("dart:io");
 #import("testing/dart/test_runner.dart");
 #import("testing/dart/test_options.dart");
 #import("testing/dart/test_suite.dart");
@@ -81,10 +82,11 @@ 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));
       }
     }