JsNumber.toFixed either needs more overloads or needs @DoNotAutobox

[niloc132]

Discovered via https://stackoverflow.com/questions/57244451/bug-jsnumber-tofixed-returns-different-values-in-superdev-and-js

When calling JsNumber.toFixed with an int value (as would seem to be the sanest option), the int is boxed to Integer, which isn't treated as a number by the underlying ToInteger -> ToNumber -> ToPrimitive (see https://www.ecma-international.org/ecma-262/5.1/#sec-15.7.4.5 etc).

Adding elemental2.core.JsNumber.toFixed.digits to core's integer-entities.txt only results in this logged message at build time:

SEVERE: Unused integer entities: [elemental2.core.JsNumber.toFixed.digits]

Some proposed solutions that seem to make sense on face value (though I haven't tried to fully implement any of them yet):

• In the same way as integer-entities.txt, provide a way to mark some parameter as @DoNotAutobox, so that if truly any value is allowed, an int/float/etc is passed directly to JS without boxing. It might be reasonable to have this be the default, since native methods aren't likely to specially handle java.lang.Integer, etc.
• In the extern diffs, specify that this method might take Object, or might take Number, allowing the regular integer-entities.txt functionality to work here.
• Permit jsinterop.generator.visitor.IntegerEntitiesConverter in google/jsinterop-generator to act on any supertype of Double/double, so that overloads are generated automatically.

[jDramaix]

I think we should fix the extern file for this method. It should accept a Number and not an Object.

Then we can use interger-entities.txt to turn the parameter to int.

[zbynek]

PR in Closure created