In Debian, the users have reported that Qt 5.7.1 update breaks Plasma: https://bugs.debian.org/843250

While investigating that bug, I was able to write a small enough test case:

import QtQuick 2.0

Text {
    property var someText
    text: "Hello, " + someText.text

    Component.onCompleted: {
        console.log(someText);
        console.log(someText.hasOwnProperty);
    }
}

import QtQuick 2.0

MyLabel {
    someText: Text { text: "world!" }
}

With Qt 5.7.1, I get this output:

$ qml ./main.qml
qml: QQuickText(0x556bfb6ed290)
qml: undefined

So hasOwnProperty method is unavailable. The same goes for any other built-in method, like isPrototypeOf, constructor and so on.

In Qt 5.6.1 (and 5.7.0), it worked fine:

$ qml ./test.qml 
qml: QQuickText(0x5592b042cf30)
qml: function() { [code] }