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

runtime/bin/extensions_macos.cc

Index: runtime/bin/extensions_macos.cc
diff --git a/runtime/bin/extensions_macos.cc b/runtime/bin/extensions_macos.cc
index 71a95f6240d93c7496bfb76dbf2da763f77a2036..d147d9d7e47d667dddbf3415f1b4e629a683f5b6 100644
--- a/runtime/bin/extensions_macos.cc
+++ b/runtime/bin/extensions_macos.cc
@@ -7,7 +7,7 @@
 
 void* Extensions::LoadExtensionLibrary(const char* library_path,
                                        const char* extension_name) {
-  const char* strings[5] = { library_path, "/lib",
+  const char* strings[] = { library_path, "/lib",
                              extension_name, ".dylib", NULL };
   char* library_file = Concatenate(strings);
   void* lib_handle = dlopen(library_file, RTLD_LAZY);