Upgrading our project to the latest version of Qt has been set back due to a change that breaks the Sugar.js library which we use extensively. The author of the library has investigated the issue and concluded that:

an update to QML has made all native methods configurable: false, meaning that any attempt to override them results in a TypeError

The author did suggest a workaround and I don't expect this to be addressed any time soon (but some official response would be nice), so this issue report is, in a broader scope, mostly to raise awareness about breaking changes in QML's JS engine, and that it's being used with non-QML JavaScript libraries (related: QTBUG-22356). To quote the author:

the changes QML have instituted are not part of spec, and could potentially interfere with polyfills, so should be treated as a bug

See the quoted discussion here: https://github.com/andrewplummer/Sugar/issues/551

The issue might be present with 5.6 as well, I can't test it out at the moment.