Deprecate injected arguments for signal handlers

The following code snippet does not work correctly when running it with the code pre-compiled with qmlcachegen:

import QtQuick 2.0
import QtQuick.Window 2.1
Window {
    width: 800
    height: 600
    visible: true
    signal sig(string arg)
    onSig: {
        print("SIG", arg);
        var arg = "bar";
    }
    Component.onCompleted: {
        sig("foo");
    }
}

$ qmlcachegen sig.qml -o sig.qmlc
$ qml sig.qml
qml: SIG undefined
$ QML_DISABLE_DISK_CACHE=1 qml sig.qml
qml: SIG foo

The behavior is correct when disk cache is disabled.

Commenting the 'var arg = "bar"' line fixes the issue, but leaving it seems to prevent the code from resolving the signal parameter "arg" when executing the function.