args: deprecate `callback` and replace with `parse`

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan Originally opened as dart-lang/sdk#20079

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callback doesn't seem to provide much benefit over just accessing the value from the results. If it could perform validation and parsing into other types (e.g. int) and throw nice error messages, I think it would be more useful. Example:

Old:

  parser.addOption('x', callback: (x) => print('x: $x'));   parser.addOption('y', callback: (y) => print('y: $y'));

  var results = parser.parse(arguments);

  var x = int.parse(results['x']);   var y = int.parse(results['y']);   print('x + y: ${x + y}');

New:

  parser.addOption('x', parse: int.parse);   parser.addOption('y', parse: int.parse);

  var results = parser.parse(arguments);

  int x = results['x'];   int y = results['y'];   print('x: $x');   print('y: $y');   print('x + y: ${x + y}');

If --x or --y are not formatted as integers, this would lead to something like:

  foo.dart: --foo value "xyz" is invalid: FormatException: ...

Of course you could throw whatever error you want, for example you could use matchers:

  parser.addOption('foo', parse: (s) {     var matchState = {};     if(!isNumeric.matches(s, matchState) {       throw isNumeric.describeMismatch(s, new StringDescription(), matchState);     }     return num.parse(s);   });

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd

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Added Area-Pkg, Pkg-Args, Triaged labels.