Fix analyzer hints and warnings in test/.

test/global/binstubs/outdated_snapshot_test.dart

 // Copyright (c) 2014, 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.
 
-import 'dart:io';
-
 import 'package:path/path.dart' as p;
 import 'package:pub/src/io.dart';
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 import 'utils.dart';
 
@@ skipping 31 matching lines @@
     process.shouldExit();
 
     d.dir(cachePath, [
       d.dir('global_packages/foo/bin', [
         d.binaryMatcherFile('script.dart.snapshot', isNot(equals(
             readBinaryFile(testAssetPath('out-of-date.snapshot')))))
       ])
     ]).validate();
   });
 }