Set concurrency to 1 in Configuration in debug mode.

lib/src/runner/configuration.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.
 
 library test.runner.configuration;
 
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'package:args/args.dart';
@@ skipping 135 matching lines @@
 
   /// The set of platforms on which to run tests.
   List<TestPlatform> get platforms =>
       _options["platform"].map(TestPlatform.find).toList();
 
   /// Parses the configuration from [args].
   ///
   /// Throws a [FormatException] if [args] are invalid.
   Configuration.parse(List<String> args)
       : _options = _parser.parse(args) {
-    _concurrency = _options['concurrency'] == null
-        ? _defaultConcurrency
-        : _wrapFormatException('concurrency', int.parse);
+    if (pauseAfterLoad) {
+      _concurrency = 1;
+    } else if (_options['concurrency'] == null) {
+      _concurrency = _defaultConcurrency;
+    } else {
+      _concurrency = _wrapFormatException('concurrency', int.parse);
+    }
 
     if (_options["name"] != null && _options["plain-name"] != null) {
       throw new FormatException(
           "--name and --plain-name may not both be passed.");
     }
   }
 
   /// Runs [parse] on the value of the option [name], and wraps any
   /// [FormatException] it throws with additional information.
   _wrapFormatException(String name, parse(value)) {
     try {
       return parse(_options[name]);
     } on FormatException catch (error) {
       throw new FormatException('Couldn\'t parse --$name "${_options[name]}": '
           '${error.message}');
     }
   }
 }