code: public String toJson(JsonElement jsonElement) {
      StringWriter writer = new StringWriter();
      toJson(jsonElement, writer);
      return writer.toString();
    }
docs: Converts a tree of {@link JsonElement}s into its equivalent JSON representation.
  
  @param jsonElement root of a tree of {@link JsonElement}s
  @return JSON String representation of the tree
  @since 1.4