Issue #2110467 by Berdir: Add first(), get($index) and possibly other methods to ListInterface.

core/lib/Drupal/Core/Config/Schema/Sequence.php

core/lib/Drupal/Core/Entity/Query/Sql/Tables.php

core/lib/Drupal/Core/Field/FieldItemList.php

core/lib/Drupal/Core/Field/FieldItemListInterface.php

core/lib/Drupal/Core/TypedData/ListInterface.php

core/lib/Drupal/Core/TypedData/Plugin/DataType/ItemList.php

...ules/comment/lib/Drupal/comment/CommentFormController.php

...ent_translation/Tests/ContentTranslationSyncImageTest.php

...s/datetime/lib/Drupal/datetime/Tests/DateTimeItemTest.php

...es/editor/lib/Drupal/editor/Tests/EditorFileUsageTest.php

...ules/field/lib/Drupal/field/Tests/FieldValidationTest.php

...dules/field_ui/lib/Drupal/field_ui/Form/FieldEditForm.php

...les/hal/lib/Drupal/hal/Normalizer/FieldItemNormalizer.php

...modules/hal/lib/Drupal/hal/Normalizer/FieldNormalizer.php

core/modules/link/lib/Drupal/link/Tests/LinkItemTest.php

.../modules/rdf/lib/Drupal/rdf/Tests/StandardProfileTest.php

core/modules/rest/lib/Drupal/rest/Tests/NodeTest.php

...system/lib/Drupal/system/Tests/Entity/EntityFieldTest.php