Add slice/slice-assignment operators.

[lrhn]

A lot of library operations on strings and lists would be more easily expressed using a Python (etc.) like "slice operator".

 T operator [:]({int start, int end});
 void operator[:]=(var value, {int start, int end});

This would replace String.substring with [:], make it easy to add List.subList/replace List.getRange with [:], use [:]= to replace insertRange.

Both parameters are optional, and usually default to start = 0 and end = length of sequence if omitted.

[DartBot]

This comment was originally written by @seaneagan

---

this is a duplicate of issue #983

[floitschG]

Thanks.

---

Added Duplicate label.
Marked as being merged into #983.