Helpers are emitted at the top of the file before any imports. This way these helpers can not be imported using external module imports.

import 'reflect-metadata';
@inject
class House {
}

var __metadata = this.__metadata || (typeof Reflect === "object" && Reflect.metadata) || function () { };
require('reflect-metadata');

that also applies to __extends, __decorate, __param as well

See #2249 (comment) for more details