MutationEvents are deprecated in DOM3 Events, and removed in DOM4 (see http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#mutationevent) in favor of MutationObservers (http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#mutation-observers). The inspector's text editor makes use of DOMNodeInserted, DOMNodeInsertedIntoDocument, and DOMSubtreeModified. It would be ideal if that usage could be replaced with MutationObservers.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.