public ImmutableList<String> getFieldDeclarations() {
    return bitmaskFields.stream()
        .map(field -> "private " + field.type + " " + field.name + ";")
        .collect(toImmutableList());
  }
Returns code to declare any fields needed to track which properties have been set. Each line in
the returned list should appear on a line of its own.
value/src/main/java/com/google/auto/value/processor/BuilderRequiredProperties.java