Issue #2694009 by heddn: MigrationInterface doesn't include get()

core/modules/file/src/Plugin/migrate/source/d7/File.php

...le/tests/src/Kernel/Migrate/d6/FileMigrationTestTrait.php

core/modules/migrate/src/MigrateExecutable.php

core/modules/migrate/src/Plugin/Migration.php

core/modules/migrate/src/Plugin/MigrationInterface.php

core/modules/migrate/src/Plugin/migrate/id_map/Sql.php

.../modules/migrate/src/Plugin/migrate/process/Migration.php

...es/migrate/src/Plugin/migrate/source/SourcePluginBase.php

core/modules/migrate/src/Plugin/migrate/source/SqlBase.php

core/modules/migrate/tests/src/Unit/MigrateTestCase.php

.../modules/migrate/tests/src/Unit/process/MigrationTest.php

...c/Plugin/migrate/destination/EntityFieldStorageConfig.php

.../modules/migrate_drupal_ui/src/MigrateUpgradeRunBatch.php

...ests/src/Kernel/Migrate/d6/MigrateUserPictureFileTest.php