Module not found: Error: Can't resolve 'core-js/modules/es.array.concat.js code example

Example 1: Module not found: Error: Can't resolve 'core-js/es7/reflect'

// Change import in polyfills.ts
import 'core-js/es7/reflect';
// for
import 'core-js/es/reflect';

Example 2: Module not found: Error: Can't resolve 'core-js/client/shim'

import 'core-js/es/symbol'