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

runtime/bin/extensions_linux.cc

Index: runtime/bin/extensions_linux.cc
diff --git a/runtime/bin/extensions_linux.cc b/runtime/bin/extensions_linux.cc
index e0c65740bb0a9d2202547e6321281e72a05d521b..68e2908fc33e2f277108c5646f7c1f5066e1ba5f 100644
--- a/runtime/bin/extensions_linux.cc
+++ b/runtime/bin/extensions_linux.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, ".so", NULL };
   char* library_file = Concatenate(strings);
   void* lib_handle = dlopen(library_file, RTLD_LAZY);