code: public void add(String property, JsonElement value) {
      members.put(property, value == null ? JsonNull.INSTANCE : value);
    }
docs: Adds a member, which is a name-value pair, to self. The name must be a String, but the value
  can be an arbitrary JsonElement, thereby allowing you to build a full tree of JsonElements
  rooted at this node.
  
  @param property name of the member.
  @param value the member object.