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

tools/testing/dart/test_suite.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.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 640 matching lines @@
     Directory tempDir = new Directory(tempDirPath);
     if (!tempDir.existsSync()) {
       // Dartium tests can be run with no build step, with no output directory.
       // This special case builds the build directory that should be there.
       var buildPath = tempDirPath.split('/');
       tempDirPath = buildPath[0];
       if (tempDirPath == '') {
         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();
       }
     }
     tempDirPath = new File(tempDirPath).fullPathSync().replaceAll('\\', '/');
     return TestUtils.mkdirRecursive(tempDirPath,
                                     Strings.join(generatedTestPath, '/'));
   }
 
@@ skipping 548 matching lines @@
  * $noCrash tests are expected to be flaky but not crash
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
 """;
     print(report);
    }
 }