Test Scripts: Fix Windows path error in layout tests.

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 631 matching lines @@
           var dartFlags = ['--ignore-unrecognized-flags'];
           if (configuration["checked"]) {
             dartFlags.add('--enable_asserts');
             dartFlags.add("--enable_type_checks");
           }
           dartFlags.addAll(vmOptions);
           args.add('--dart-flags=${Strings.join(dartFlags, " ")}');
         }
         args.add(htmlPath);
         if (expectedOutput != null) {
-          args.add('--out-expectation=$expectedOutput');
+          args.add('--out-expectation=${expectedOutput.toNativePath()}');
         }
       }
       commands.add(new Command('python', args));
 
       // Create BrowserTestCase and queue it.
       var testCase = new BrowserTestCase('$suiteName/$testName',
           commands, configuration, completeHandler, expectations,
           optionsFromFile['isNegative']);
       doTest(testCase);
     }
@@ skipping 731 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);
    }
 }