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Since Angular 8, the CLI has transformed decorator metadata to a form that can be used by the Angular dependency injector without the TDZ limitations of Typescript's decorator metadata emit feature. As a result, a JIT application compiled with the CLI no longer requires the reflect metadata polyfill that was provided by `core-js`. This polyfill was also the last remaining usage of the `core-js` package within `@angular-devkit/build-angular` which allows the `core-js` package to also be removed. Refs: https://github.com/angular/angular-cli/pull/14473 & https://github.com/angular/angular/pull/37382 BREAKING CHANGE: Reflect metadata polyfill is no longer automatically provided in JIT mode Reflect metadata support is not required by Angular in JIT applications compiled by the CLI. Applications built in AOT mode did not and will continue to not provide the polyfill. For the majority of applications, the reflect metadata polyfill removal should have no effect. However, if an application uses JIT mode and also uses the previously polyfilled reflect metadata JavaScript APIs, the polyfill will need to be manually added to the application after updating. To replicate the previous behavior, the `core-js` package should be manually installed and the `import 'core-js/proposals/reflect-metadata';` statement should be added to the application's `polyfills.ts` file.