code: public Scriptable getBuiltinPrototype(Builtins type) {
          BaseFunction func = getBuiltinCtor(type);
          Object proto = func != null ? func.getPrototypeProperty() : null;
          return proto instanceof Scriptable ? (Scriptable) proto : null;
      }
docs: Get the cached built-in object prototype from this scope with the
  given <code>type</code>. Returns null if {@link #cacheBuiltins()} has not
  been called on this object.
  @param type the built-in type
  @return the built-in prototype