Simplify Command classes.

tools/testing/dart/test_progress.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.
 
 import 'dart:convert' show JSON;
 import 'dart:io';
 
+import 'command.dart';
+import 'command_output.dart';
 import 'configuration.dart';
 import 'expectation.dart';
 import 'path.dart';
 import 'summary_report.dart';
 import 'test_runner.dart';
 import 'utils.dart';
 
 /// Controls how message strings are processed before being displayed.
 class Formatter {
   /// Messages are left as-is.
@@ skipping 648 matching lines @@
 }
 
 String _buildSummaryEnd(int failedTests) {
   if (failedTests == 0) {
     return '\n===\n=== All tests succeeded\n===\n';
   } else {
     var pluralSuffix = failedTests != 1 ? 's' : '';
     return '\n===\n=== ${failedTests} test$pluralSuffix failed\n===\n';
   }
 }