Issue #2065721 by tim.plunkett: Add a dedicated @Block plugin annotation.

...rupal/aggregator/Plugin/Block/AggregatorCategoryBlock.php

...ib/Drupal/aggregator/Plugin/Block/AggregatorFeedBlock.php

...lib/Drupal/custom_block/Plugin/Block/CustomBlockBlock.php

core/modules/block/lib/Drupal/block/Annotation/Block.php

...dules/block/lib/Drupal/block/Plugin/Type/BlockManager.php

...Drupal/block_test/Plugin/Block/TestBlockInstantiation.php

...est/lib/Drupal/block_test/Plugin/Block/TestCacheBlock.php

...st/lib/Drupal/block_test/Plugin/Block/TestHtmlIdBlock.php

.../lib/Drupal/block_test/Plugin/Block/TestXSSTitleBlock.php

...book/lib/Drupal/book/Plugin/Block/BookNavigationBlock.php

...t/lib/Drupal/comment/Plugin/Block/RecentCommentsBlock.php

...forum/lib/Drupal/forum/Plugin/Block/ActiveTopicsBlock.php

...es/forum/lib/Drupal/forum/Plugin/Block/NewTopicsBlock.php

...nguage/lib/Drupal/language/Plugin/Block/LanguageBlock.php

core/modules/menu/lib/Drupal/menu/Plugin/Block/MenuBlock.php

.../node/lib/Drupal/node/Plugin/Block/RecentContentBlock.php

...ules/node/lib/Drupal/node/Plugin/Block/SyndicateBlock.php

...les/search/lib/Drupal/search/Plugin/Block/SearchBlock.php

...rtcut/lib/Drupal/shortcut/Plugin/Block/ShortcutsBlock.php

...Drupal/statistics/Plugin/Block/StatisticsPopularBlock.php