Evaluation looks fine.

I didn't add new syntax nodes for the new import assertions.
And I didn't add support for import assertions in the JS parser.
E.g. import obj from "./something.json" assert { type: "json" };.
It's currently a stage 3 proposal.

TypeScript added support for the #name in obj syntax, which we didn't support in the JS parser.
So I implemented the ECMAScript proposal first. (It's a stage 4 proposal).

The #name in obj syntax is weird.
For context: name in obj tests whether the string-value contained in the variable name is present as a field in obj.
However, #name in obj tests whether the private field #name (from the current class) is present in obj.
That also means that two #name in obj expressions in two different classes will refer to two different #name fields.

It appears to be the only situation across JavaScript where an identifier does not refer to a value, but instead refers to the existence of a field.
So the implementation is somewhat hacky, but that comes from it being a special case in the language itself.