Make TestUtils.copyDirectory() more robust by ensureing arguments are a native path

dart/tools/testing/dart/test_suite.dart

Index: dart/tools/testing/dart/test_suite.dart
diff --git a/dart/tools/testing/dart/test_suite.dart b/dart/tools/testing/dart/test_suite.dart
index 3bb3b21c93c6a0d49e3465cb9133da7e874bb201..1df99ad1f4f660ac5557cad2f632e6ef0381ef30 100644
--- a/dart/tools/testing/dart/test_suite.dart
+++ b/dart/tools/testing/dart/test_suite.dart
@@ -1889,11 +1889,14 @@ class TestUtils {
   }
 
   static Future copyDirectory(String source, String dest) {
+    source = new Path(source).toNativePath();
+    dest = new Path(dest).toNativePath();
+
     var executable = 'cp';
     var args = ['-Rp', source, dest];
     if (Platform.operatingSystem == 'windows') {
       executable = 'xcopy';
-      args = [source, dest, '/e'];
+      args = [source, dest, '/e', '/i'];

[kustermann, 2014/01/07 14:52:27]

Either dest has to end with a backslash or '/i' has to be passed, otherwise
xcopy will ask a Y/N question.

     }
     return Process.run(executable, args).then((ProcessResult result) {
       if (result.exitCode != 0) {