title: Can't find variable: Dimensions
body: <p>When I try to use:</p>
  
  <pre><code>const {width, height} = Dimensions.get('window');
  </code></pre>
  
  <p>On a react native component, I get a weird error never seen before:</p>
  
  <p>Can't find variable: Dimensions </p>
  
  <p><a href="https://i.stack.imgur.com/fBrUg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/fBrUg.png" alt="screenshot of the error"></a></p>
  
  <p>Have someone face this issue before? </p>
  
  <pre><code>"dependencies": {
  "expo": "^22.0.0",
  "react": "16.0.0-beta.5",
  "react-native": "https://github.com/expo/react-native/archive/sdk-22.0.2.tar.gz"
  </code></pre>
  
  <p>},</p>