static Executable of(
      ExecutableElement executableElement, ImmutableSet<String> optionalParameters) {
    return new Executable(executableElement, optionalParameters);
  }
A wrapper for an {@link ExecutableElement}, representing a static method or a constructor. This
wrapper then allows us to attach additional information, such as which parameters have Kotlin
defaults.
value/src/main/java/com/google/auto/value/processor/Executable.java