7 files declare their use of MigrateSourceInterface in Drupal 8

ContentEntityTest.php in core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php

migrate.api.php in core/modules/migrate/migrate.api.php

Hooks provided by the Migrate module.

MigrateStubTest.php in core/modules/migrate/tests/src/Unit/MigrateStubTest.php

migrate_prepare_row_test.module in core/modules/migrate/tests/modules/migrate_prepare_row_test/migrate_prepare_row_test.module

Test module for testing the migration source plugin prepareRow() exception handling.

MigrationStateUnitTest.php in core/modules/migrate_drupal/tests/src/Unit/MigrationStateUnitTest.php

MigrationTest.php in core/modules/migrate/tests/src/Unit/MigrationTest.php

Contains \Drupal\Tests\migrate\Unit\MigrationTest.

SourcePluginBase.php in core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php