pkg/scheduled_test: fix analyzer hints

lib/scheduled_process.dart

 // Copyright (c) 2013, 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 scheduled_test.scheduled_process;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
-import 'package:stack_trace/stack_trace.dart';
-
 import 'scheduled_stream.dart';
 import 'scheduled_test.dart';
 import 'src/utils.dart';
 import 'src/value_future.dart';
 
 /// A class representing a [Process] that is scheduled to run in the course of
 /// the test. This class allows actions on the process to be scheduled
 /// synchronously. All operations on this class are scheduled.
 ///
 /// Before running the test, either [shouldExit] or [kill] must be called on
@@ skipping 297 matching lines @@
     schedule(() {
       return _exitCode.then((exitCode) {
         if (expectedExitCode != null) {
           expect(exitCode, equals(expectedExitCode));
         }
       });
     }, "waiting for process '$description' to exit");
     _scheduledExitTask = currentSchedule.tasks.contents.last;
   }
 }