Does anyone know if any ES2015 features are optimized yet? When last I checked, just having an unused `let` statement in a function made V8 bail out to the slo

Does anyone know if any ES2015 features are optimized yet? When last I checked, just having an unused `let` statement in a function made V8 bail out to the slow compiler, making ES2015 off-limits for anything nontrivial. But that was some months ago.