Issue #2318087 by tim.plunkett: Replace ['input'] with FormState::getUserInput().

core/includes/form.inc

core/lib/Drupal/Core/Form/FormBuilder.php

core/lib/Drupal/Core/Form/FormState.php

core/lib/Drupal/Core/Form/FormStateInterface.php

core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php

core/modules/book/src/Form/BookAdminEditForm.php

core/modules/comment/src/Form/ConfirmDeleteMultiple.php

core/modules/editor/src/Form/EditorImageDialog.php

core/modules/editor/src/Form/EditorLinkDialog.php

core/modules/field/field.module

core/modules/file/file.module

.../modules/file/src/Plugin/Field/FieldWidget/FileWidget.php

core/modules/image/src/Form/ImageStyleEditForm.php

...odules/image/src/Plugin/Field/FieldWidget/ImageWidget.php

core/modules/menu_ui/src/MenuForm.php

core/modules/simpletest/src/Form/SimpletestTestForm.php

core/modules/system/src/Controller/FormAjaxController.php

core/modules/system/src/Tests/Batch/ProcessingTest.php

...modules/system/src/Tests/Form/ElementsTableSelectTest.php

core/modules/system/src/Tests/Form/FormTest.php