fix(app): toString() does not exist in modular API, use .name property

Author: mikehardy

packages/app/lib/FirebaseApp.js

@@ -73,7 +73,7 @@ export default class FirebaseApp {
   }
 
   toString() {
-    warnIfNotModularCall(arguments);
+    warnIfNotModularCall(arguments, '.name property');
     return this.name;
   }
 }