[dart2wasm] Consider treating `JSValue` a primitive type and unbox it in fields & function signatures

[mkustermann]

We should do an experiment and see how code size (e.g. on ACX ) & performance would look like if we treated JSValue as a primitive just like bool/int/double values, more specifically: We can pass it around unboxed as externref and store in structs as externref and box when needed.

We get externrefs by calling JS methods and we use them by calling JS methods - so maybe it would avoid unnecessary boxing.

It may also lower live data and therefore GC pressure by not having all these boxes in the heap.

/cc @osa1

[osa1]

IIRC we discussed this in the past (before js_interop) and the usual problem with passing things unboxed and boxing when necessary is that their identities change as you box them.

One idea that had came up in the past was maintaining a weak map from JS objects to their identities. The box classes would then index the map with the JS objects to get the identities, or create a new identity if the object is not already there.

That said, I just realized that js_interop documentation mentions in a note that == should be used for JS value equality:

Similarly, identical may return different results for the same JS value depending on the compiler. Use == to check for equality of two JS types instead.

So I guess it's now fine to give the same JS object a new identity every time we box it.

[mkustermann]

I don't think we need to guarantee identities for JSValue. I assume nobody would rely on their identity anyway, also not on their identity hash, exactly because we can have the same JS value in many JSValue boxes and one can therefore not use them in an identity Map as a key.