Clean up hints after a new hint was introduced

pkg/analyzer_cli/lib/src/perf_report.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer_cli.src.perf_report;
 
 import 'dart:convert' show JsonEncoder;
-import 'dart:io' show File, Platform;
+import 'dart:io' show Platform;
 
 import 'package:analyzer/src/generated/utilities_general.dart'
     show PerformanceTag;
 import 'package:analyzer/task/model.dart' show AnalysisTask;
 import 'package:analyzer_cli/src/error_formatter.dart';
 import 'package:analyzer_cli/src/options.dart' show CommandLineOptions;
 
 const _JSON = const JsonEncoder.withIndent("  ");
 
 bool _isCheckedMode = () {
@@ skipping 84 matching lines @@
 }
 
 class _TaskRow {
   final String name;
   final int time;
   final int count;
   _TaskRow(this.name, this.time, this.count);
 
   Map toJson() => <String, dynamic>{'name': name, 'time': time, 'count': count};
 }