Equivalent to:
ZoneCallback registered = this.registerUnaryCallback(action);
if (runGuarded) return (arg) => this.runUnaryGuarded(registered, arg);
return (arg) => thin.runUnary(registered, arg);
Source
ZoneUnaryCallback/*<R, T>*/ bindUnaryCallback/*<R, T>*/(
/*=R*/ action(/*=T*/ argument), { bool runGuarded: true });