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