code: @Override
      public void setText(String text) {
          if (headingToggle != null) {
              if (text == null) text = "";
              String s = headingToggle.getInnerHTML();
              int p = s.indexOf('<');
              if (p >= 0) s = text + s.substring(p);
              else s = text;
              headingToggle.setInnerHTML(s);
          } else if (!isInstance(getElement())) {
              if (headerPanel != null) {
                  super.remove(headerPanel);
                  headerPanel = null;
              }
              header.setText(text);
          }
      }
docs: Sets the text on the header element