anchor.diff:          final arguments = step["arguments"]
               .map((argument) => expandVariables(argument, builder))
               .toList();
  -        final String namedConfiguration = arguments
  +        final namedConfiguration = arguments
               .firstWhere((argument) => (argument as String).startsWith("-n"))
               .substring(2);
           if (namedConfiguration.contains(",")) {
  
positive.code:       orElse: () => "master");
  }
  
  class ResolvedConfigurations {
    final Set<String> configurationNames;
    final Set<String> builders;
    ResolvedConfigurations(this.configurationNames, this.builders);
  }
  
  /// Finds the named configuration to test according to the test matrix
  /// information and the command line options.
  ResolvedConfigurations resolveNamedConfigurations(
      List<String> branches,
      List<dynamic> buildersConfigurations,
      String requestedBranch,
      List<String> requestedNamedConfigurations,
      String requestedBuilder) {
    Set<String> namedConfigurations = {};
    Set<String> builders = {};
    bool foundBuilder = false;
    for (final builderConfiguration in buildersConfigurations) {
      for (final builder in builderConfiguration["builders"]) {
        if (requestedBuilder != null && builder != requestedBuilder) {
  
negative.code:     return null;
    }
    if (requestedBuilder != null &&
        requestedNamedConfigurations.isEmpty &&
        testedConfigurations.isEmpty) {
      stderr.writeln("error: Builder $requestedBuilder isn't testing any named "
          "configurations");
      return null;
    }
    if (requestedNamedConfigurations.isNotEmpty) {
      var hasUntestedConfiguration = false;
      for (final requestedConfiguration in requestedNamedConfigurations) {
        if (!testedConfigurations.containsKey(requestedConfiguration)) {
          final builder = requestedBuilder != null
              ? "builder $requestedBuilder"
              : "any builder";
          stderr.writeln("error: The named configuration "
              "$requestedConfiguration isn't tested on $builder");
          hasUntestedConfiguration = true;
        }
      }
      if (hasUntestedConfiguration) {
        return null;
      }
    }
  
    return testedConfigurations;
  }
  
  /// Locates the merge base between head and the [branch] on the given [remote].
  