9 files declare their use of MarkupTrait

BigPipeMarkup.php in core/modules/big_pipe/src/Render/BigPipeMarkup.php
FieldFilteredMarkup.php in core/lib/Drupal/Core/Field/FieldFilteredMarkup.php
FilteredMarkup.php in core/modules/filter/src/Render/FilteredMarkup.php
HtmlTest.php in core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
IFrameMarkup.php in core/modules/media/src/IFrameMarkup.php
Markup.php in core/lib/Drupal/Core/Render/Markup.php
SafeMarkupTest.php in core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
Contains \Drupal\Tests\Component\Utility\SafeMarkupTest.
TwigMarkupInterfaceTest.php in core/tests/Drupal/KernelTests/Core/Theme/TwigMarkupInterfaceTest.php
Contains \Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest.
ViewsRenderPipelineMarkup.php in core/modules/views/src/Render/ViewsRenderPipelineMarkup.php

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