anchor.diff:  
     List<String> _constructReproducingCommandArguments(Map config) {
       var arguments = new List<String>();
  -    for (var configKey in config.keys) {
  -      if (!_blacklistedOptions.contains(configKey)) {
  -        for (var option in _options) {
  -          var configValue = config[configKey];
  -          // We only include entries of [conf] if we find an option for it.
  -          if (configKey == option.name && configValue != option.defaultValue) {
  -            var isBooleanOption = option.type == 'bool';
  -            // Sort by length, so we get the shortest variant.
  -            var possibleOptions = new List.from(option.keys);
  -            possibleOptions.sort((a, b) => (a.length < b.length ? -1 : 1));
  -            var key = possibleOptions[0];
  -            if (key.startsWith('--')) {
  -              // long version
  -              arguments.add(key);
  -              if (!isBooleanOption) {
  -                arguments.add("$configValue");
  -              }
  -            } else {
  -              // short version
  -              assert(key.startsWith('-'));
  -              if (!isBooleanOption) {
  -                arguments.add("$key$configValue");
  -              } else {
  -                arguments.add(key);
  -              }
  -            }
  -          }
  -        }
  +    for (var option in _options) {
  +      var name = option.name;
  +      if (!config.containsKey(name) || _blacklistedOptions.contains(name)) {
  +        continue;
  +      }
  +      var value = config[name];
  +      if (config[name] == option.defaultValue ||
  +          (name == 'packages' &&
  +              value ==
  +                  TestUtils.dartDirUri.resolve('.packages').toFilePath())) {
  +        continue;
  +      }
  +      shortest(String a, String b) => a.length <= b.length ? a : b;
  +      var key = option.keys.reduce(shortest);
  +      if (option.type == 'bool') {
  +        arguments.add(key);
  +      } else if (key.startsWith('--')) {
  +        // long version
  +        arguments.add(key);
  +        arguments.add("$value");
  +      } else {
  +        // short version
  +        assert(key.startsWith('-'));
  +        arguments.add("$key$value");
         }
       }
       return arguments;
  
positive.code:     if (selectors.containsKey('observatory_ui')) {
        if (selectors.length == 1) {
          configuration['packages'] = TestUtils.dartDirUri
            .resolve('runtime/observatory/.packages').toFilePath();
        } else {
          // Make a new configuration whose selectors map only contains
          // observatory_ui, and remove the key from the original selectors.
          // The only mutable value in the map is the selectors, so a
          // shallow copy is safe.
          var observatoryConfiguration = new Map.from(configuration);
          observatoryConfiguration['selectors'] =
            {'observatory_ui': selectors['observatory_ui']};
          selectors.remove('observatory_ui');
  
          // Set the packages flag.
          observatoryConfiguration['packages'] = TestUtils.dartDirUri
            .resolve('runtime/observatory/.packages').toFilePath();
  
          // Return the expansions of both configurations. Neither will reach
          // this line in the recursive call to _expandConfigurations.
  
negative.code:     if (configuration['package_root'] == null &&
          configuration['packages'] == null) {
        configuration['packages'] =
            TestUtils.dartDirUri.resolve('.packages').toFilePath();
      }
  
      // Expand the architectures.
  