title: flatMap, flat, flatten doesn't exist on type any[]
body: <p>I'm using Chrome 70 and Chrome does add methods <code>.flatMap</code>, <code>.flatten</code>, <code>.flat</code>. So my code does run as expected. Unfortunately, TypeScript doesn't like it.</p>
  <pre><code>// data.flatMap lint error
  export const transformData = (data: any[]) =&gt; data.flatMap(abc =&gt; [
     parentObj(abc),
     ...generateTasks(abc)
  ]);
  </code></pre>
  <p>The warning I got is <code>TS2339: Property 'flatMap' does not exist on type 'any[]'.</code></p>
  <p>I'm using Angular 6, which uses Typescript ~2.9.2, and I already include <code>import 'core-js/es7/array';</code> in <code>polyfills.ts</code>.</p>
  <p>My guess is that there is no typing for these methods, and I did try to <code>npm run -dev @types/array.prototype.flatmap</code> but still not solve.</p>