Add swap macro

[gnudles]

swapping two integer variables cannot be made in function, because it alters only the local value.
fact: variable swap takes 3 lines of code.
please add swap macro that do swap instead of doing it in every place we need.

[Semvrij]

Swapping variables will become much easier when a way to destructure (#207) is added.

Current method:

int a = 1;
int b = 2;

int temp = a;
a = b;
b = temp;

With destructuring:

int a = 1;
int b = 2;

[a, b] = [b, a];

[gnudles]

Destructure does not seem like it coming soon, or ever.
I think a macro or a special syntax should be in the meantime

[ChristianKleineidam]

It's a huge part of darts language philosophy to be a concise language to learn. If special syntax would created to solve a bunch of cases like this (that aren't even important enough for the author to argue why they need it in real world programs) the special syntax would make dart more complex and harder to learn.

Core language features aren't supposed to be added to solve issues "in the mean time".

[gnudles]

You've got a point. It is just annoying to write 3 lines of code each time you need a tiny swap. That's all.