ImmutableMap<String, BuilderSpec.PropertyGetter> builderGetters() {
    return ImmutableMap.copyOf(builderGetters);
  }
Returns the set of properties that have getters in the builder. If a property is defined by an
abstract method in the {@code @AutoValue} class called {@code foo()} or {@code getFoo()} then
the name of the property is {@code foo}, If the builder also has a method of the same name
({@code foo()} or {@code getFoo()}) then the set returned here will contain {@code foo}.
value/src/main/java/com/google/auto/value/processor/BuilderMethodClassifier.java