title: firebase.firestore() is not a function when trying to initialize Cloud Firestore
body: <p>When I try to initialize Firebase Cloud Firestore, I ran into the following error: </p>
  
  <blockquote>
    <p>Uncaught TypeError: <strong>WEBPACK_IMPORTED_MODULE_0_firebase</strong>.firestore is not a function</p>
  </blockquote>
  
  <p>I installed firebase with <code>npm install firebase --save</code> previously.</p>
  
  <pre><code>import * as firebase from 'firebase';
  import router from '../router';
  
  const config = {
          apiKey: "a",
          authDomain: "a",
          databaseURL: "a",
          projectId: "a",
          storageBucket: "a",
          messagingSenderId: "a"
  };
  if(!firebase.apps.length){
    firebase.initializeApp(config);
    let firestore = firebase.firestore();
  }
  </code></pre>