Call me crazy, but this seems like a typo
@@ -142,7 +142,7 @@ const Plugin = {
} );
- block.removeAttribute( 'data-fragment-index' )
+ block.removeAttribute( 'data-fragment-index' );
block.setAttribute( 'data-line-numbers', Plugin.serializeHighlightSteps( [ highlightSteps[0] ] ) );
}