public String getBuiltToBuilder() {
      return builtToBuilder;
    }
A method to convert the built type back into a builder. Unfortunately Guava collections don't
have this (you can't say {@code myImmutableMap.toBuilder()}), but for other types such as
{@code @AutoValue} types this is {@code toBuilder()}.
value/src/main/java/com/google/auto/value/processor/PropertyBuilderClassifier.java