title: use of undeclared type 'Date' Xcode 9 Swift 4
body: <p>This is my code in xcode 9 playground </p>
  
  <pre><code>class Tutorial: Codable {
      let title: String
      let author: String
      let editor: String
      let type: String
      let publishDate: Date
  
      init(title: String, author: String, editor: String, type: String, publishDate: Date) {
          self.title = title
          self.author = author
          self.editor = editor
          self.type = type
          self.publishDate = publishDate
      }
  }
  </code></pre>
  
  <p>its showing error use of undeclared type 'Date' </p>