Incorrect realization of SetItem() functions

Hello,
The following code behaves incorrectly (inconsistent with the standard and other engines) in Chakra due to incorrect realization of SetItems() functions:

```
var y = 0;
var t = [1,2,3];

var mp = new Proxy([], {
get: function (oTarget, sKey) {
print("get " + sKey.toString());
y = y + 1;
print("y = " + y);
if(y == 2){
var temp = [];
t.length = 20;
for (var i=0; i<20; i++)
t[i] = 0x10;
return 5;
}
if(y > 2) {
return oTarget[sKey];
}
return oTarget[sKey] || undefined;
//return oTarget[sKey] || oTarget.getItem(sKey) || undefined;
},
set: function (oTarget, sKey, vValue) {
print("set " + sKey);
if (sKey in oTarget) { return false; }
return oTarget[sKey] = vValue;
},

});

function f(a){
print(a);
}

t.length = 4;
t.__proto__ = mp;

f(...t);
```

the 't[i] = 0x10;' in get() should call set() when i is undefined, however, in Chakra, t[i] is assigned directly.