Fix Windows build.

runtime/bin/dartutils.cc

Index: runtime/bin/dartutils.cc
diff --git a/runtime/bin/dartutils.cc b/runtime/bin/dartutils.cc
index 08a8452eba349cab62e0f9fda888ef7acff185a9..db8e1fa1dbf5c02233efc3075861514a760bef21 100644
--- a/runtime/bin/dartutils.cc
+++ b/runtime/bin/dartutils.cc
@@ -238,7 +238,7 @@ char* DartUtils::DirName(const char* url) {
   if (slash == NULL) {
     return strdup(url);
   } else {
-    return strndup(url, slash - url + 1);
+    return StringUtils::StrNDup(url, slash - url + 1);
   }
 }