Allow attributes to be set on all entities via one hook (hook_entity_view_alter())

In Drupal 7, and before this patch got committed, it was effectively impossible to set an entity-specific attribute (e.g. one that includes its ID) on all entities (i.e. across all entity types: node, taxonomy term, custom block, and so on). You could not even set a foo="bar" attribute on all entities!

We solved this by having attributes set in hook_entity_view_alter() be inherited (merged) with existing attributes.
(In fact, we solved this generically: in template_preprocess(), the basic preprocessing function that runs for all rendered things, we made sure that if we're rendering a so-called render element, and if that render element has any #attributes set, then we merge those with the attributes that will be passed to the template or theme function: with $variable['attributes']).

Drupal 7

/**
 * Implements hook_preprocess_HOOK() for node.tpl.php.
 */
function edit_preprocess_node(&$variables) {
  $node = $variables['elements']['#node'];
  $variables['attributes']['data-edit-entity'] = 'node/' . $node->nid;
}

/**
 * Implements hook_preprocess_HOOK() for taxonomy-term.tpl.php.
 */
function edit_preprocess_taxonomy_term(&$variables) {
  $term = $variables['elements']['#term'];
  $variables['attributes']['data-edit-entity'] = 'taxonomy_term/' . $term->id();
}

Drupal 8

/**
 * Implements hook_entity_view_alter().
 */
function edit_entity_view_alter(&$build, EntityInterface $entity, EntityViewDisplayInterface $display) {
  $build['#attributes']['data-edit-entity'] = $entity->entityType() . '/' . $entity->id();
}