Create

display.js

var map;
window.initMap = function(){
  map = new google.maps.Map(document.getElementById('map'), {
    center: {lat: -34.397, lng: 150.644},
    zoom: 8
  });
}

display.libraries.yml

google.maps:
  version: VERSION
  js:
    https://maps.googleapis.com/maps/api/js?key=YourAPIkey&callback=initMap:
      type: external
      attributes:
        async: true
        defer: true

LocationBlock.php

<?php

/**
 * @file
 * Contains \Drupal\tilt_display\Plugin\Block\LocationBlock.
 */

namespace Drupal\tilt_display\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;
use Drupal\Core\Link;

/**
 * Provides a 'LocationBlock' block.
 *
 * @Block(
 *  id = "location_block",
 *  admin_label = @Translation("Location block"),
 * )
 */
class LocationBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $form['google_maps_api_key'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('Google Maps API key'),
      '#description' => $this->t('Goto the Google Developer Console to generate the API key https://console.developers.google.com.'),
      '#default_value' => isset($this->configuration['google_maps_api_key']) ? $this->configuration['google_maps_api_key'] : '',
      '#maxlength' => 255,
      '#size' => 64,
      '#weight' => '0',
    );

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    $this->configuration['google_maps_api_key'] = $form_state->getValue('google_maps_api_key');
  }

  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = [];

    $url = Url::fromRoute('contact.site_page');
    $link = Link::fromTextAndUrl(t("Contact us"), $url);
    $link = $link->toRenderable();
    $link['#attributes'] = array('class' => array('contact button'));

    $contactTitle = t('Meet us !');
    $address = t('255 Chaussée de Waterloo, 1060 Brussels, Belgium');

    $output = [
      '#theme' => 'tilt_display_location',
      '#google_maps_api_key' => $this->configuration['google_maps_api_key'],
      '#contact_link' => render($link),
      '#contact_title' => $contactTitle,
      '#address' => $address,
      // @todo review pros and cons between attached libraries and attached jss
      // @todo review how to pass api key to tilt_display.libraries.yml
      /*
      '#attached' => [
        'js' => [
          'https://maps.googleapis.com/maps/api/js?key='.$this->configuration['google_maps_api_key'],
          drupal_get_path('module', 'tilt_display') . '/js/tilt_display.js',
        ]
      ],
      */
      '#attached' => array(
        'library' => array(
            'tilt_display/tilt_display',
            'tilt_display/google.maps',
        ),
      ),
    ];
    $build['location_block_google_maps_api_key']['#markup'] = render($output);

    return $build;
  }

}