Refactor URI processing of native extensions directive #import("dart-ext:foo").

runtime/bin/extensions_win.cc

Index: runtime/bin/extensions_win.cc
diff --git a/runtime/bin/extensions_win.cc b/runtime/bin/extensions_win.cc
index 4fdaba83631454099f9971dfb82b7bd6ba23164b..52edf59d59da26a43a61b3ece1b2447db87d688e 100644
--- a/runtime/bin/extensions_win.cc
+++ b/runtime/bin/extensions_win.cc
@@ -6,7 +6,7 @@
 
 void* Extensions::LoadExtensionLibrary(const char* library_path,
                                        const char* extension_name) {
-  const char* strings[5] = { library_path, "/", extension_name, ".dll", NULL };
+  const char* strings[] = { library_path, "/", extension_name, ".dll", NULL };
   char* library_file = Concatenate(strings);
   void* lib_handle = LoadLibrary(library_file);
   free(library_file);