The flag --experimental-strip-types has been enabled by default. Only syntax that dont require transformation is allowed. Syntax that require transformation are enum, namespaces (namespaces containing types only are allowed) and legacy modules.You need to pass the flag --experimental-transform-types to enable it. Decorators will soon be supported by V8 since they are a stage 3 proposal. We decided not to polyfill it but to wait the V8 relase.