8 files declare their use of CommandInterface

EditorDialogSave.php in core/modules/editor/src/Ajax/EditorDialogSave.php
HighlightCommand.php in core/modules/views/src/Ajax/HighlightCommand.php
ReplaceTitleCommand.php in core/modules/views/src/Ajax/ReplaceTitleCommand.php
SetFormCommand.php in core/modules/views_ui/src/Ajax/SetFormCommand.php
SetSubtreesCommand.php in core/modules/toolbar/src/Ajax/SetSubtreesCommand.php
ShowButtonsCommand.php in core/modules/views/src/Ajax/ShowButtonsCommand.php
TriggerPreviewCommand.php in core/modules/views/src/Ajax/TriggerPreviewCommand.php
UpdateSelectionCommand.php in core/modules/media_library/src/Ajax/UpdateSelectionCommand.php

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