Visit Facebook (yeah, I know). Locate a post's reply block and focus on it. Start typing. Press backspace key to delete what you have typed. Nothing happens.
I could reproduce with nightly r180087. We need to bisect to find out what change broke this.
<rdar://problem/19837231>
Turns out that this regressed with http://trac.webkit.org/changeset/179429
*** This bug has been marked as a duplicate of bug 141859 ***
Oops, it's my fault. Thank you for investigating it! After investigating the cause, it seems that, when window.Symbol is exposed, Object.getOwnPropertySymbols and the latest ES6 iterator interface are also required. So I'm now fixing this issue by hiding window.Symbol object to user web pages. It allow inspector to continue to use Symbol and its' already used.