default String finalAutoValueClassName() {
      throw new UnsupportedOperationException();
    }
The fully-qualified name of the last class in the {@code AutoValue} hierarchy. For an
{@code @AutoValue} class {@code foo.bar.Baz}, this will be {@code foo.bar.AutoValue_Baz}. The
class may be generated by an extension, which will be the current extension if the {@code
isFinal} parameter to {@link AutoValueExtension#generateClass} is true and the returned
string is not {@code null}.

<p>For compatibility reasons, this method has a default implementation that throws an
exception. The AutoValue processor supplies an implementation that behaves as documented.
value/src/main/java/com/google/auto/value/extension/AutoValueExtension.java