Fix test.dart in the case of running dartium tests on Windows without building dart.

tools/testing/dart/test_suite.dart

Index: tools/testing/dart/test_suite.dart
diff --git a/tools/testing/dart/test_suite.dart b/tools/testing/dart/test_suite.dart
index 9ddfdb4019a3851450b4124da85e9aa6b8642faf..c3ea82f31e2e8b6ce7df48fdafa8e4e4a543e452 100644
--- a/tools/testing/dart/test_suite.dart
+++ b/tools/testing/dart/test_suite.dart
@@ -658,10 +658,12 @@ class StandardTestSuite implements TestSuite {
         throw new Exception(
             'Non-relative path to build directory in test_suite.dart');
       }
-      buildPath.removeRange(0, 1);
-      if (buildPath.last() == '') buildPath.removeLast();
-      buildPath.addAll(generatedTestPath);
-      generatedTestPath = buildPath;
+      if (buildPath.length > 1) {
+        buildPath.removeRange(0, 1);
+        if (buildPath.last() == '') buildPath.removeLast();
+        buildPath.addAll(generatedTestPath);
+        generatedTestPath = buildPath;
+      }
       tempDir = new Directory(tempDirPath);
       if (!tempDir.existsSync()) {
         tempDir.createSync();