simplify: dedupe boilerplate and remove dead code across all layers

lib/widget/debug/cache_view.dart

@@ -1,4 +1,3 @@
-import 'dart:async';
 import 'dart:convert';
 import 'package:filesize/filesize.dart';
 import 'package:flutter/material.dart';
@@ -85,11 +84,3 @@ class _CacheViewState extends State<CacheView> {
     ),
   );
 }
-
-extension FutureExtension<T> on Future<T> {
-  bool isCompleted() {
-    final completer = Completer<T>();
-    then(completer.complete).catchError(completer.completeError);
-    return completer.isCompleted;
-  }
-}