It seems that when the qmljs plugin was ported over from qt-creator, the OSX specific files were left out.
fileutils_mac.mm
fileutils_mac.h
autoreleasepool.h
Note: I simply copied these over from HEAD of qt-creator git.
Upon inspection of the license headers of other files in that directory, this appears to match what was done before.
This does include a modification to CMakeLists.txt to add the object-c++ file to compilation.