code: public RFuture<String> getText (Keyboard.TextType textType, String label, String initialValue,
                                    String ok, String cancel) {
      return RFuture.failure(new Exception("getText not supported"));
    }
docs: Requests a line of text from the user. On platforms that have only a virtual keyboard, this
  will display a text entry interface, obtain the line of text, and dismiss the text entry
  interface when finished. Note that HTML5 and some Java backends do not support customization
  of the OK and Cancel labels. Thus those platforms will ignore the supplied labels and use
  their mandatory labels.
  
  @param textType the expected type of text. On mobile devices this hint may be used to display a
  keyboard customized to the particular type of text.
  @param label a label to display over the text entry interface, may be null.
  @param initialValue the initial value to display in the text input field, may be null.
  @param ok the text of the button which will deliver a {@code true} result and be placed in
  "OK" position for the platform.
  @param cancel the text of the button that will deliver a {@code false} result and be placed in
  "Cancel" position.
  
  @return a future which provides the text when it becomes available. If the user cancels the
  text entry process, null is supplied. Otherwise the entered text is supplied.