Minor tweak.

packages/coreutils/src/url.ts

@@ -102,9 +102,8 @@ namespace URLExt {
   export
   function isLocal(url: string): boolean {
     const { protocol } = parse(url);
-    const lower = url.toLowerCase();
 
-    return lower.indexOf(protocol) !== 0 && lower.indexOf('//') !== 0;
+    return url.toLowerCase().indexOf(protocol) !== 0 && url.indexOf('//') !== 0;
   }
 
   /**