title: Ionic2 can't build app with --prod flag
body: <p>I have developed an Ionic2 app and it runs smoothly when I build it with </p>
  
  <pre><code>ionic cordova build android
  </code></pre>
  
  <p>but when I try to build it with --prod flag i.e.</p>
  
  <pre><code>ionic cordova build android --prod
  </code></pre>
  
  <p>it gives following error</p>
  
  <pre><code>[18:09:32]  build prod started ...
  [18:09:32]  clean started ...
  [18:09:32]  clean finished in 63 ms
  [18:09:32]  copy started ...
  [18:09:32]  deeplinks started ...
  [18:09:33]  deeplinks finished in 1.04 s
  [18:09:33]  ngc started ...
  [18:10:00]  typescript error
          src\app\app.module.ts(287,19): Error during template compile of 'AppModule' Expression form not supported.
  
  Error: The Angular AoT build failed. See the issues above
  at D:\aafia\node_modules\@ionic\app-scripts\dist\aot\aot-compiler.js:237:55
  at step (D:\aafia\node_modules\@ionic\app-scripts\dist\aot\aot-compiler.js:32:23)
  at Object.next (D:\aafia\node_modules\@ionic\app-scripts\dist\aot\aot-compiler.js:13:53)
  at fulfilled (D:\aafia\node_modules\@ionic\app-scripts\dist\aot\aot-compiler.js:4:58)
  </code></pre>
  
  <p><strong>Note</strong>: I am using ionic-cli 3.20.0</p>