Key on String instead of index int

[lukepighetti]

Matching on string will work for indexes, but it will also work for arbitrary tags such as 'target:foo'. Matching on index only works for numbers, and you end up having to maintain a map of string keys to fake indexes. This is for lists with dynamic length and widgets of dynamic height where the target may not yet be rendered because it's below the fold. Any feedback is appreciated!

[jerrywell]

using numbers as key is because it will count on the order to forecast which direction and what distance we may move to. And the index is also matching the default sliver builder's index. if you use it for only few rows with dynamic height, you can use SingleChildScrollView with GlobalKey.