Astral code points are garbled

[mandel59]

Characters in Unicode supplementary planes (such as 𠮟, 𩸽 and 𠀋) are garbled. To copy and paste them also brings garbled results.

Test PDF
XeLaTeX source code

Expected:

Actual:

Environment: OS X 10.9.4, Firefox 31.0, PDF.js in mozilla/pdf.js master

[Snuffleupagus]

Using: Windows 7 (64-bit), Firefox Nightly 34 (buildID: 20140816030208) with HWA on, and PDF.js 1.0.700; this is how the file renders for me:

As far as I can tell, copying has never worked properly.
The missing character unfortunately appears to be a regression from PR #5017.