title: How to specify environment via `ng build` in Angular 6 app
body: <p>In Angular 5,
  we could generate build for different environments using</p>
  
  <pre><code>ng build --prod --env=uat
  </code></pre>
  
  <p>After migration to Angular 6 the above command throws error</p>
  
  <pre><code>Unknown option: '--env'
  </code></pre>