Add optional starting position to String.replaceFirst.

[whesse]

Just as String.contains allows one to start a search at the middle of a string, it would be nice if we could replace starting at a certain offset.
This slightly simplifies "Replace the last _ in a string with /".

Of course, working RegExp in replace would also allow this.

Also, should RegExp.matchFirst allow starting at an index in the string?

[whesse]

We still have nothing like String.replaceLast, which I keep needing.
Even if it is hard to do with a RegExp pattern, it should be doable with
a string pattern.

Otherwise, the starting position for replaceFirst would be great. And a starting position for RegExp.firstMatch.

[lrhn]

Added Library-Core label.

[lrhn]

I don't think we will add String.replaceLast. It depends on searching backwards, and Pattern doesn't have that functionality (and we are not likely to add it).

That means that the special casing for searching backwards for a string has to be in String.replaceLast, and other patterns can't get the same special treatment. That's undesirable.
On the other hand, lastIndexOf has the same problem, so this argument isn't consistent :)

[lrhn]

The start-index on replaceFirst has been added (thanks srawlins).
We are not planning on adding replaceLast.

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Added Fixed label.