ImmutableSet<ExecutableElement> builderAbstractMethods() {
      if (builderAbstractMethods == null) {
        builderAbstractMethods = abstractMethods(builderTypeElement, processingEnv);
      }
      return builderAbstractMethods;
    }
Representation of an {@code AutoValue.Builder} class or interface.
value/src/main/java/com/google/auto/value/processor/BuilderSpec.java