Math.parse{Int,Double} could be constructors

[DartBot]

This issue was originally filed by @seaneagan

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It would be more consistent/logical to have Math.parseInt and Math.parseDouble as constructors:

interface int {
  int.parse(String s); // could use "fromString" but it's a bit long
  //...
}

interface num {
  num.parse(String s);
  //...
}

interface double {
  double.parse(String s);
  //...
}

[DartBot]

This comment was originally written by @seaneagan

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It might make more sense as static methods in order to be able to handle invalid Strings and nulls intelligently (similar to issue #2870):

interface int {
  // returns ifInvalid if s is null or is incorrectly formatted
  static int parse(String s, [int ifInvalid]);
  //...
}

interface num {
  // returns ifInvalid if s is null or is incorrectly formatted
  static num parse(String s, [int ifInvalid]);
  //...
}

interface double {
  // returns ifInvalid if s is null or is incorrectly formatted
  static double parse(String s, [int ifInvalid]);
  //...
}

[sethladd]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[gbracha]

Removed Area-Language label.
Added Area-Library label.

[DartBot]

This comment was originally written by @seaneagan

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If going the static method route, I think it would be more orthogonal to just always return null on invalid input rather than providing an "ifInvalid" parameter.

In any case, the solution should be consistent with issue #2870.

[floitschG]

We are pretty happy with the current solution:
int int.parse(String source, {int radix, int onError(String source)})
double double.parse(String source, { double onError(String source)})

Still missing num.parse and I will open another bug for that: issue #8237.

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Added Fixed label.