UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:60.0) Gecko/20100101 Firefox/60.0

Steps to reproduce the problem:
1. 
chrome.storage.local.set({'a': '\udf4f'}, function() {
  chrome.storage.local.get('a', function() {
    console.log(d['a'].charCodeAt(0).toString(16));
  });
});

What is the expected behavior?
console output:  df4f  ,which is an "INVALID CHARACTER" (\df4f)

What went wrong?
it outputs fffd , which is "REPLACEMENT CHARACTER" (\ufffd)

Did this work before? No 

Does this work in other browsers? N/A

Chrome version: 65.0.3325.181  Channel: stable
OS Version: 
Flash Version: Shockwave Flash 29.0 r0

I have tested this on firefox 59.It works correctly.