[dart2js] Update web/native/ tests for `@Native()` classes.

[rakudrama]

We have a requirement that a @Native class implements JavaScriptObject.
This requirement comes from ensuring that static js-interop works as an alternative to access the same objects also declared via @Native classes in the same program.

Most tests in this directory were written before static js-interop and do not meet the requirement of implementing JavaScriptObject.

Since few of the tests mix @Native and static js-interop access or interfaces relevant to static js-iterop, I suggest we update these tests as needed.