Should dart2wasm wrapper lists around JS values implement equality based on their JS values?

[srujzs]

We have a number of wrapper lists for typed arrays and arrays in the dart2wasm implementation of dart:js_interop conversions. Specifically, toDart conversions wrap the JS value with a Dart wrapper.

Currently, == is unimplemented in each of these wrappers - except for JSArrayBufferImpl, which uses the underlying JS value for comparison - which then means we just check if the wrappers themselves are equal when we use ==. Should we instead implement == in each of these to check the underlying value? If not, should we change JSArrayBufferImpl's implementation of ==?

Making them check the underlying value would make this more consistent with dart2js/ddc except for the one case there where a cast list is introduced when calling JSArray.toDart (a.toDart == a.toDart would naturally then not be equal in that case).

cc @osa1

[mkustermann]

The == implementation would be based on JS object identity rather than structural equality. Since objects that compare equal should also have the same hashCode, it would mean the hash code of those objects is also based on JS identity hashes (which don't exist).

Now that's where an issue comes in: By default the two dart wrappers will get different identity hashes assigned to them. We cannot ask for an identity hash from the JS object.

That brings us back to #55823: Assign 0 (or any other fixed hash code) to them or store in JS. I'm not sure if modifying the JS array objects by putting more properties on them is a good idea.

So if we made them equal, we may need to use a fixed hash code.

[srujzs]

Okay, that makes sense.

So it seems like we're okay with equality being on the underlying JS value in these wrappers, we just need to make hashCode either 0 or some unique value derived from the JS value after #55823 is fixed. Is that correct?