title: Can't read annotation when use Symfony Validator as standalone
body: <p><strong>I get this error:</strong></p>
  
  <blockquote>
    <p>Message: "[Semantical Error] The annotation
    "@Symfony\Component\Validator\Constraints\Length" in property
    User::$name does not exist, or could not be auto-loaded."</p>
  </blockquote>
  
  <p><strong>This is the code on Github <a href="https://github.com/symfony/Validator" rel="nofollow noreferrer">https://github.com/symfony/Validator</a></strong></p>
  
  <pre><code>use Symfony\Component\Validator\Validation; 
  use Symfony\Component\Validator\Constraints as Assert;
  
  class User {
      /**
       * @Assert\Length(min = 3)
       * @Assert\NotBlank
       */
      private $name;
  
      /**
       * @Assert\Email
       * @Assert\NotBlank
       */
      private $email;
  
      public function __construct($name, $email)
      {
          $this-&gt;name = $name;
          $this-&gt;email = $email;
      }
  
      /**
       * @Assert\True(message = "The user should have a Google Mail account")
       */
      public function isGmailUser()
      {
          return false !== strpos($this-&gt;email, '@gmail.com');
      } 
  }
  
  $validator = Validation::createValidatorBuilder()
      -&gt;enableAnnotationMapping()
      -&gt;getValidator();
  
  $user = new User('John Doe', 'john@example.com');
  
  $violations = $validator-&gt;validate($user);
  </code></pre>
  
  <p><strong>How can I fix this problem ?</strong></p>