num parse(String input, [num onError(String input)])

Parses a string containing a number literal into a number.

The method first tries to read the input as integer (similar to int.parse without a radix). If that fails, it tries to parse the input as a double (similar to double.parse). If that fails, too, it invokes onError with input, and the result of that invocation becomes the result of calling parse.

If no onError is supplied, it defaults to a function that throws a FormatException.

For any number n, this function satisfies identical(n, num.parse(n.toString())) (except when n is a NaN double with a payload).

Source 

static num parse(String input, [num onError(String input)]) {
  String source = input.trim();
  // TODO(lrn): Optimize to detect format and result type in one check.
  num result = int.parse(source, onError: _returnIntNull);
  if (result != null) return result;
  result = double.parse(source, _returnDoubleNull);
  if (result != null) return result;
  if (onError == null) throw new FormatException(input);
  return onError(input);
}