Fix analyzer hints and warnings in test/.

test/run/forwards_signal_posix_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.
 
 // Windows doesn't support sending signals.
 @TestOn('!windows')
 
 import 'dart:io';
 
+import 'package:scheduled_test/scheduled_test.dart';
+
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 const _catchableSignals = const [
   ProcessSignal.SIGHUP,
   ProcessSignal.SIGTERM,
   ProcessSignal.SIGUSR1,
   ProcessSignal.SIGUSR2,
   ProcessSignal.SIGWINCH,
 ];
@@ skipping 26 matching lines @@
 
     pub.stdout.expect("ready");
     for (var signal in _catchableSignals) {
       pub.signal(signal);
       pub.stdout.expect(signal.toString());
     }
 
     pub.kill();
   });
 }