This would would be correct if it was in a method of ve.ui.MWBackTool,
but this is actually a different class ve.ui.MWBackCommand (they are
both in the same file), which doesn't have a 'toolbar' property.
This partially reverts commit 4984c5ffbb
"Avoid using mw.ArticleTarget methods on global ve.init.target in tools".
Bug: T279613
Change-Id: Ia5e80e34cc5021295639cf18b3c324d2821aecf5