anchor.diff:  
     void operator []=(K key, V value) {
       if (key == null) {
  -      JS('', '# = null', key);
  +      key = JS('', 'null');
       } else if (JS<bool>('!', '#[#] !== #', key, dart.extensionSymbol('_equals'),
           dart.identityEquals)) {
         key = putLinkedMapKey(key, _keyMap);
  
positive.code: 
    V? operator [](Object? key) {
      if (key == null) {
        JS('', '# = null', key);
      } else if (JS<bool>('!', '#[#] !== #', key, dart.extensionSymbol('_equals'),
          dart.identityEquals)) {
        @notNull
  
negative.code: 
    V? remove(Object? key) {
      if (key == null) {
        key = JS('', 'null');
      } else if (JS<bool>('!', '#[#] !== #', key, dart.extensionSymbol('_equals'),
          dart.identityEquals)) {
        @notNull
  