Fix analyzer hints and warnings in test/.

test/ascii_tree_test.dart

 // Copyright (c) 2012, 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 lock_file_test;
 
 import 'package:pub/src/ascii_tree.dart' as tree;
-import 'package:pub/src/utils.dart';
 import 'package:test/test.dart';
 
 main() {
   group('tree.fromFiles', () {
     test('no files', () {
       expect(tree.fromFiles([]), equals(""));
     });
 
     test('up to ten files in one directory are shown', () {
       var files = [
@@ skipping 217 matching lines @@
     |-- g.dart
     |-- h.dart
     |-- i.dart
     |-- j.dart
     |-- k.dart
     '-- l.dart
 """));
   });
 
 }