code: @NonNull
      @Override
      public MutableDictionary remove(@NonNull String key) {
          if (key == null) { throw new IllegalArgumentException("key cannot be null."); }
  
          synchronized (lock) {
              internalDict.remove(key);
              return this;
          }
      }
docs: Removes the mapping for a key from this Dictionary
  
  @param key the key.
  @return The self object.