anchor.diff:                  doElse$17 = false;
                   return (() {
                       $dart$core.Object item = Map.$_$lookup($outer$ceylon$language$Map, key);
  -                    if ((!(null == item)) && (item == $package$missing)) {
  +                    if ($dart$core.identical(item, $package$$new$Missing$instance)) {
                           return null;
                       } else {
                           return $capture$Map$mapItems$mapping.f(key, item);
  
positive.code:                 doElse$18 = false;
                  return (() {
                      $dart$core.Object item = Map.$_$lookup($outer$ceylon$language$Map, key);
                      if ((!(null == item)) && (item == $package$missing)) {
                          return $default;
                      } else {
                          return $capture$Map$mapItems$mapping.f(key, item);
  
negative.code: 
  emptyMap_ get emptyMap => $package$emptyMap;
  
  class Missing {
      void _$init$Missing([$dart$core.int $bitmap]) {
          if (0 != ($bitmap & 1)) {}
      }
      Missing.instance() : this.instance$d(1);
      Missing.instance$d([$dart$core.int $bitmap]) {
          _$init$Missing($bitmap | 1);
      }
  }
  Missing _$s$Missing$instance;
  
  Missing get $package$$new$Missing$instance => _$s$Missing$instance != null ? _$s$Missing$instance : _$s$Missing$instance = new Missing.instance();
  
  Missing get $new$Missing$instance => $package$$new$Missing$instance;
  
  $dart$core.Object $package$max([Iterable values]) {
      Iterator it = values.iterator();
  