Fix d8 path for Windows.

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 182 matching lines @@
     if (name == '') {
       name = executablePath;
     }
 
     TestUtils.ensureExists(name, configuration);
     return name;
   }
 
   String get d8FileName {
     var suffix = getExecutableSuffix('d8');
-    var d8Dir = '${TestUtils.dartDir()}/third_party/d8';
-    var d8 = '$d8Dir/${Platform.operatingSystem}/d8$suffix';
+    var d8Dir = TestUtils.dartDir().append('third_party/d8');
+    var d8Path = d8Dir.append('${Platform.operatingSystem}/d8$suffix');
+    var d8 = d8Path.toNativePath();
     TestUtils.ensureExists(d8, configuration);
     return d8;
   }
 
   String get jsShellFileName {
     var executableSuffix = getExecutableSuffix('jsshell');
     var executable = 'jsshell$executableSuffix';
     var jsshellDir = '${TestUtils.dartDir()}/tools/testing/bin';
     return '$jsshellDir/$executable';
   }
@@ skipping 1708 matching lines @@
  * $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
  * $compileErrorSkip tests are skipped on browsers due to compile-time error
 """;
     print(report);
    }
 }