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).