Remove unused imports of system libraries. (#76436)

packages/flutter_tools/test/commands.shard/hermetic/build_ios_framework_test.dart

@@ -4,8 +4,6 @@
 
 // @dart = 2.8
 
-import 'dart:io';
-
 import 'package:file/memory.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/platform.dart';

packages/flutter_tools/test/integration.shard/debugger_stepping_test.dart

@@ -4,8 +4,6 @@
 
 // @dart = 2.8
 
-import 'dart:io';
-
 import 'package:file/file.dart';
 
 import '../src/common.dart';

packages/flutter_tools/test/integration.shard/expression_evaluation_test.dart

@@ -4,8 +4,6 @@
 
 // @dart = 2.8
 
-import 'dart:io';
-
 import 'package:file/file.dart';
 import 'package:matcher/matcher.dart';
 

packages/flutter_tools/test/integration.shard/flutter_attach_test.dart

@@ -4,8 +4,6 @@
 
 // @dart = 2.8
 
-import 'dart:io';
-
 import 'package:file/file.dart';
 import 'package:vm_service/vm_service.dart';
 

packages/flutter_tools/test/web.shard/debugger_stepping_web_test.dart

@@ -4,8 +4,6 @@
 
 // @dart = 2.8
 
-import 'dart:io';
-
 import 'package:file/file.dart';
 
 import '../integration.shard/test_data/stepping_project.dart';