Add "last" setter to List.

Add "last" setter to List.

This allows assigning directly as `list.last = value` instead of
having to use the longer `list[list.length - 1] = value`.
It's just an abbreviation, like `list.last` is for reading.

If you use a List as a stack (which is a perfectly good use of it),
modifying the last element like this is a natural operation.

R=sgjesse@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37255

[Søren Gjesse, 2014-06-10 07:14:49]

lgtm

[Lasse Reichstein Nielsen, 2014-06-12 08:11:52]

Committed patchset #1 manually as r37255 (presubmit successful).

[nweiz, 2014-06-12 23:03:58]

https://codereview.chromium.org/315173005/diff/1/pkg/collection/lib/wrappers....
File pkg/collection/lib/wrappers.dart (right):

https://codereview.chromium.org/315173005/diff/1/pkg/collection/lib/wrappers....
pkg/collection/lib/wrappers.dart:128: _listBase.last = value;
This will cause an analyzer warning on versions of the SDK that don't support
[List.last=] in the core libraries. It may be a good idea to increase the SDK
constraint in the pubspec to avoid that.