title: Without `from` option PostCSS could generate wrong source map or do not find Browserslist config
body: <blockquote>
    <p>Witout <code>from</code> option PostCSS could generate wrong source map or do not
    find Browserslist config. Set it to CSS file path or to <code>undefined</code> to
    prevent this warning</p>
  </blockquote>
  
  <p>ionic3 project, when I running <code>ionic corodva bulid ios --prod</code> showing this warning for me.</p>
  
  <p><strong>ionic info</strong></p>
  
  <pre><code>cli packages: (/usr/local/lib/node_modules)
  
      @ionic/cli-utils  : 1.19.0
      ionic (Ionic CLI) : 3.19.0
  
  global packages:
  
      cordova (Cordova CLI) : 7.1.0 
  
  local packages:
  
      @ionic/app-scripts : 3.1.7
      Cordova Platforms  : android 6.3.0 ios 4.4.0
      Ionic Framework    : ionic-angular 3.9.2
  
  System:
  
      ios-deploy : 1.9.2 
      Node       : v6.11.3
      npm        : 3.10.10 
      OS         : macOS Sierra
      Xcode      : Xcode 9.2 Build version 9C40b 
  
  Environment Variables:
  
      ANDROID_HOME : not set
  
  Misc:
  
      backend : pro
  </code></pre>
  
  <p>UPDATE:</p>
  
  <p>I tried to update my npm to 5.5.1 and node to 8.6.0, but still showing.</p>