title: Why is ES7/array polyfill needed despite the tsconfig target is set to ES5
body: <p>I have the following settings in the<code>tsconfig.json</code>. I added <code>"es2017"</code> to use <code>Array.includes</code>.:</p>
  
  <pre><code>{
    "compilerOptions": {
      "lib": [
        "es6",
        "es2017",
        "dom"
      ],
      "module": "es6",
      "target": "es5"
    }
  }
  </code></pre>
  
  <p>Now, I realized, that I have to add <code>import 'core-js/es7/array';</code> to the <code>polyfills.ts</code>, to use <code>Array.includes</code> also for the Internet Explorer 11. The <code>target</code> in the <code>tsconfig.json</code> is set to <code>es5</code>, which does not have <code>Array.includes</code>.</p>
  
  <p>Why do I need to add the polyfill? </p>