Specify behavior of Math.pow

Library doc needs to specify the behavior of Math.pow, both for doubles and integers. In the absence of documentation Dart attempts to implement behavior defined in ECMA-262 15.8.2.13.

[lrhn]

The ES5 standard is pretty arbitrary. It doesn't match any of the three power functions in IEEE-754-2008.
I think we should just use the IEEE pow function:

For the pow function (integral exponents get special treatment):
 pow(x, ±0) is 1 for any x (even a zero, quiet NaN, or infinity)
 pow(±0, y) is ±∞ and signals the divideByZero exception for y an odd integer <0
 pow(±0, −∞) is +∞ with no exception
 pow(±0, +∞) is +0 with no exception
 pow(±0, y) is +∞ and signals the divideByZero exception for finite y<0 and not an odd integer
 pow(±0, y) is ±0 for finite y>0 an odd integer
 pow(±0, y) is +0 for finite y>0 and not an odd integer
 pow(−1, ±∞) is 1 with no exception
 pow(+1, y) is 1 for any y (even a quiet NaN)
 pow(x, y) signals the invalid operation exception for finite x<0 and finite non-integer y.

where the exception of the last line is handled by returning NaN.

This matches ECMAScript Math.pow except for the two next-to-last cases (-1 ** [-]Infinity, 1 ** anything), and it's likely to have an efficient implementation in native libraries.

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cc @floitschG.
Set owner to @lrhn.

[floitschG]

Sounds good to me.
If I read this correctly we will guarantee an integer result for x:int, y:int and y>= 0. For all other input we return a double.

[lrhn]

Pow is now documented. It matches the cases above, and specifies some more special cases.
VM matches documentation, dart2js may not (it wasn't running lib/math tests at all).

[lrhn]

Added Fixed label.