TableCellIterator throws OutOfBounds error on while(iterator.next()) {}

[dmukhg]

The ParentNodeMixin's getChildAt method has an arrayIndex Out of bounds check. This prevents repeated calls to iterator.next() work properly.

if you do
`var iterator = new TableCellIterator(tableNode);
var cellNode;

while ((cellNode = iterator.next()) !== null) {
// some operation
}`

You get an un-handled error.

I will be creating a pull request with a failing test shortly.