String toText() {
    ImmutableMap<String, Object> vars = toVars();
    return parsedTemplate().evaluate(vars);
  }
Returns the result of substituting the variables defined by the fields of this class (a
concrete subclass of TemplateVars) into the template returned by {@link #parsedTemplate()}.
value/src/main/java/com/google/auto/value/processor/TemplateVars.java