pkg/test: remove unused imports

test/runner/browser/firefox_test.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.
 
 @TestOn("vm")
 
 import 'dart:async';
-import 'dart:io';
 
 import 'package:test/test.dart';
 import 'package:test/src/runner/browser/firefox.dart';
 import 'package:test/src/util/io.dart';
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as shelf_io;
 import 'package:shelf_web_socket/shelf_web_socket.dart';
 
 import '../../utils.dart';
 
@@ skipping 115 matching lines @@
     });
   });
 
   test("reports an error in onExit", () {
     var firefox = new Firefox("http://dart-lang.org",
         executable: "_does_not_exist");
     expect(firefox.onExit, throwsA(isApplicationException(startsWith(
         "Failed to start Firefox: No such file or directory"))));
   });
 }