Remove "unnecessary" imports in dev/ (#81004)

dev/tools/test/common.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:dev_tools/stdio.dart';

dev/tracing_tests/test/image_painting_event_test.dart

@@ -7,7 +7,6 @@ import 'dart:convert' show jsonEncode;
 import 'dart:developer' as developer;
 import 'dart:ui' as ui;
 
-import 'package:flutter/painting.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:vm_service/vm_service.dart';