specify behavior of ToStringAs* on non-doubles.

[floitschG]

toStringAsFixed, toStringAsExponential and toStringAsPrecision are defined on the num-interface. When invoked on integers should/can they convert to double first, or do they need to be implemented on integer?

I believe the right solution would be to move toStringAsFixed, toStringAsExponential and toStringAsPrecision from num to double interface, but that is not possible while int/double duality is not solved in the Javascript Dart compilers.
Consequently, I believe we should translate anu num to double before invoking toStringAsFixed, toStringAsExponential and toStringAsPrecision.

[lrhn]

We currently convert to double. That seems useful, since we don't give a way to parse the result back to an integer.

We could allow toStringAsExponential to work on integers that cannot be represented as doubles, but for now I think we should leave it as-is. I'll document this.

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Set owner to @lrhn.
Added Accepted label.

[floitschG]

Correct.
As of the latest discussions we tend to favor 'toDouble()' inside toStringAs* even though that means that bignums will potentially converted to "Infinity".

[lrhn]

Documented as converting to double.

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Added Fixed label.