title: Adding property to nested objects in Flow causes errors
body: <p>I can add a property to a flat object in Flow without any errors.</p>
  
  <p><strong><a href="https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgDCcAdhnmWALxgDeqYjYAzjAK4DmAXCxgE4BLEhwA0DMAEgMAjDDw9m-IaNQBfANzps+Im0VwAtsTIUM1IqXKUAZHVQSAFngCGAEzx8Akgecd5vQWExDXQoNhIAY2lSMFc4AGVDPAwHZQB1GQdjKwwACgjLUx5s0wBKOyYwPmS2PhIwAG17CQKTMgA6Vk4xCRbCjulZPDFKgF1NVVDwqIEYuMSDZNThDJTCPQxDErJ8-oxija298vpK6oxa+qbe1pzO9hUbvfbBuRHKplvTdqc3D29fMNxIxxmp0KhWoowBFDkY9gd9HC2mYaKcWA8eAByYDkRSY96vfyYgAqeDx71+7i8Pj8WOAAgMHHaACscBx8WpNKh5kkUulMts8jDEYLSpoeYs+SsBbDBflZcdNEA" rel="nofollow noreferrer">View demo</a></strong></p>
  
  <p>But if I add a property to a <em>nested</em> object using the same pattern, Flow throws an error:</p>
  
  <pre><code>/* @flow */
  
  type Content = {
      fields: {
          slug: string,
      },
      frontmatter: {
          title: string,
      }   
  };
  
  type CustomContent = {
      fields: {
        slug: string,
      },
      frontmatter: {
          title: string,
          // This is the new property:
          headerImage: string,
      }
  };
  
  function doSomethingWithContent(content: Content) {
      return [
          content.fields.slug,
          content.frontmatter.title,
      ];
  }
  
  function doSomethingWithCustomContent(content: CustomContent) {
      return [
          content.fields.slug,
          content.frontmatter.title,
          content.frontmatter.headerImage,
      ];
  }
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>doSomethingWithContent(customContent);
                             ^ Cannot call `doSomethingWithContent` with `customContent` bound to `content` because property `headerImage` is missing in object type [1] but exists in object type [2] in property `frontmatter`.
  </code></pre>
  
  <p><strong><a href="https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgDCcAdhnmWALxgDeqYYAkFAJZ4wAmAzgFx0NGQptxgBXAOb9uGAE6sSEgDSDGAXxWMWs0hgC2AQwzlZ-elqYZWGGHmlyFy1EzVNUagNzps+ImJlwesRkFBjUAlpsHDxmqkKikvbyippCGoLauobGeKYRQlpWNnZgMslOBVoAFngGnLkAkoYSJWWOqeruXqhQYiQAxlakYJxwAMqBeBhVjgDq1lXB5GQAFP26ofxLoQCU+YyyU2KyJGAA2s5M6yFkAHRRXNy3CU5MVxt3UDpk2Sa3RbYOmAALpeNToXoDIanUYTPRTGaKebTQj+DCBbara7LDBbNEYj4YPbmISHDDHU4XN7Y0L3diPZ7iV7vG4Ye7ffRGP4AvAqamE9lZLm5W41OqNZq8uKg9zoa4yMD9fFBQl4gIq1nhEkPGL7UpM-gAcmA5BkhtS6UYXyFOTyJMYPKNABU8GagWL6rImgYWkbgKw9BJbgArHASc2CcGedCwybTOYLTEYNbKpM7Lyx+HxpGJ1OElPqtNeIA" rel="nofollow noreferrer">View demo</a></strong></p>