This happened to me on macOS 10.12.

Steps to reproduce:

1. Check out mozilla-central revision 7befa4aa3c7f (one revision before 5433228bc018).
2. Create an Android build by running "./mach build && ./mach package" with an appropriate mozconfig.
3. Update to mozilla-central revision 5433228bc018. This changeset removed toolkit/content/widgets/toolbar.xml.
4. Re-build your Android build by running "./mach build && ./mach package" again.

Expected results:
Building and packaging should succeed.

Actual results:
Packaging fails with the following error:

 0:01.46 Traceback (most recent call last):
 0:01.46   File "/Users/mstange/code/mozilla/toolkit/mozapps/installer/packager.py", line 340, in <module>
 0:01.46     main()
 0:01.46   File "/Users/mstange/code/mozilla/toolkit/mozapps/installer/packager.py", line 334, in main
 0:01.46     copier.copy(args.destination)
 0:01.46   File "/Users/mstange/code/mozilla/python/mozbuild/mozpack/copier.py", line 430, in copy
 0:01.46     copy_results.append((destfile, f.copy(destfile, skip_if_older)))
 0:01.46   File "/Users/mstange/code/mozilla/python/mozbuild/mozpack/copier.py", line 603, in copy
 0:01.46     file.copy(deflater, skip_if_older)
 0:01.46   File "/Users/mstange/code/mozilla/python/mozbuild/mozpack/files.py", line 195, in copy
 0:01.46     shutil.copyfileobj(self.open(), dest)
 0:01.46   File "/Users/mstange/code/mozilla/python/mozbuild/mozpack/files.py", line 223, in open
 0:01.46     return open(self.path, 'rb')
 0:01.46 IOError: [Errno 2] No such file or directory: '../../../dist/bin/chrome/toolkit/content/global/bindings/toolbar.xml'
 0:01.48 make[1]: *** [stage-package] Error 1
 0:01.48 make[1]: Leaving directory `/Users/mstange/code/obj-m-android-opt/mobile/android/installer'
 0:01.48 make: *** [package] Error 2
 0:01.48 make: Leaving directory `/Users/mstange/code/obj-m-android-opt'

There's a stray symlink at objdir/dist/bin/chrome/toolkit/content/global/bindings/toolbar.xml which points to the now-nonexistent srcdir/toolkit/content/widgets/toolbar.xml file.

Deleting this symlink fixes the problem.