UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.90 Safari/537.36

Steps to reproduce the problem:
1. Sample code:

var my_array = [];
var foo = {'foo':'foo'};
var bar = {'bar':'bar'};
my_array[1] = foo;
my_array.pop();
my_array[2] = bar;

What is the expected behavior?
my_array[1] === undefined // This should be true since element on index 1 was removed by pop();

What went wrong?
my_array[1] === foo // After setting a value for index 2, the element on index 1 is present again.

Did this work before? Yes 

Chrome version: 60.0.3112.90  Channel: n/a
OS Version: 10.0
Flash Version: