9 files declare their use of AssertHelperTrait in Drupal 8

AssertHelperTrait.php in core/modules/simpletest/src/AssertHelperTrait.php

BookUninstallValidatorTest.php in core/modules/book/tests/src/Unit/BookUninstallValidatorTest.php

FieldUninstallValidatorTest.php in core/modules/field/tests/src/Unit/FieldUninstallValidatorTest.php

FilterUninstallValidatorTest.php in core/modules/filter/tests/src/Unit/FilterUninstallValidatorTest.php

ForumUninstallValidatorTest.php in core/modules/forum/tests/src/Unit/ForumUninstallValidatorTest.php

KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php

ModuleRequiredByThemesUninstallValidatorTest.php in core/tests/Drupal/Tests/Core/Extension/ModuleRequiredByThemesUninstallValidatorTest.php

RequiredModuleUninstallValidatorTest.php in core/tests/Drupal/Tests/Core/Extension/RequiredModuleUninstallValidatorTest.php

TestBase.php in core/modules/simpletest/src/TestBase.php