drupal_get_destination() is replaced by the redirect.destination service

Drupal 7:

url('path', array('query' => drupal_get_destination());

Drupal 8:

In procedural code you can access the redirect.destination service as follow:

Url::fromRoute('route_name', [], ['query' => \Drupal::service('redirect.destination')->getAsArray()]);

// or 

Url::fromRoute('route_name', [], ['query' => \Drupal::destination()->getAsArray()]);

For application-level code, such as classes that would implement ContainerInjectionInterface was introduced the RedirectDestinationTrait.
FormBase and ControllerBase already make use of it.

If your class is neither a controller or form, you can use the RedirectDestinationTrait

Example:

use Drupal\Core\Routing\RedirectDestinationTrait;

class MyClass {
  use RedirectDestinationTrait;

  /**
   * Does something.
   */
  public function doSomething() {
    // ...
    $destination = $this->getDestinationArray();
    // ...
  }

}

drupal_get_destination() remains for backwards compatibility and will be removed before 9.0.0