title: Mobx @observable is not work if its init value is undefined in react-native 0.59
body: <p>I'm migrating react-native 0.51.1 to 0.59.8. And I'm facing a problem of Mobx.</p>
  
  <p>If an @observable decorated member have assigned init value like this</p>
  
  <pre class="lang-js prettyprint-override"><code>@observable cnt = 0;
  </code></pre>
  
  <p>then it works.</p>
  
  <p>But if it's undefined,</p>
  
  <pre class="lang-js prettyprint-override"><code>@observable cnt;
  </code></pre>
  
  <p>then it doesn't work.</p>
  
  <p>I have many undefined observable stores and they worked in 0.51.0.<br>
  I want to make it work as undefined.</p>
  
  <p>Babel decorate options have been changed during migration.</p>
  
  <pre class="lang-js prettyprint-override"><code>// babel.config.js
  module.exports = {
    presets: ['module:metro-react-native-babel-preset'],
    plugins: [
      ['@babel/plugin-transform-flow-strip-types'],
      ['@babel/plugin-proposal-decorators', { 'legacy': true}],
      ['@babel/plugin-proposal-class-properties', { 'loose': true}],
    ],
  }
  </code></pre>
  
  <pre class="lang-js prettyprint-override"><code>//This doesn't work but worked in react-native 0.51.0
  import { observable } from 'mobx';
  
  export class Count {
    @observable cnt;
  
    constructor(initValue = 0) {
      this.cnt = initValue;
    }
  
    add(){
      this.cnt++;
    }
  }
  </code></pre>
  
  <pre class="lang-js prettyprint-override"><code>//This works
  import { observable } from 'mobx';
  
  export class Count {
    @observable cnt = 0;
  
    constructor(initValue = 0) {
      this.cnt = initValue;
    }
  
    add(){
      this.cnt++;
    }
  }
  </code></pre>