private boolean propertyIsNullable(String property) {
    E propertyElement = propertyElements().get(property);
    return Stream.of(propertyElement, originalPropertyType(propertyElement))
        .flatMap(ac -> ac.getAnnotationMirrors().stream())
        .map(a -> a.getAnnotationType().asElement().getSimpleName())
        .anyMatch(n -> n.contentEquals("Nullable"));
  }
True if the given property is nullable, either because its type has a {@code @Nullable} type
annotation, or because its getter method has a {@code @Nullable} method annotation.
value/src/main/java/com/google/auto/value/processor/BuilderMethodClassifier.java