Regression: Javascript array push() method doesn't work on QStringList

import QtQuick 2.0

DropArea {
    keys: ["cat"]

    Component.onCompleted: {
        console.log("My animals at start: " + keys)
        keys.push("dog")
        console.log("My animals after push: " + keys)
        keys[keys.length] = "dog"
        console.log("My animals after insert: " + keys)
    }
}