#3180. Add tests for NullableUndefineableJSAnyExtension.

[sgrekhov]

No description provided.

[eernstg]

LGTM.

@srujzs, it would be great if you could take a look at this as well!

[eernstg]

This is the most interesting case. I don't know whether or not isNull should see any difference between null and undefined.

The documentation says

Currently, there is no way to distinguish between JavaScript undefined
and JavaScript null when compiling to Wasm. Therefore, this getter
should only be used for code that compiles to JavaScript and will throw
when compiling to Wasm.

.. so there may be a need to treat compilation to Wasm and compilation to JS differently.

[srujzs]

Thanks for the cc, Erik! And thanks for adding tests, Sergey!

Yes, when compiling to Wasm, we're forced to convert to Wasm's null ref regardless of whether the JS value is undefined or null in order to maintain the same nullability semantics for interop across all compilers, so there's no way of distinguishing.

In some future, we may provide an opt-in annotation users to preserve undefined separately, but that doesn't exist yet.

So, both isNull and isUndefined need to be handled differently based on the platforms.

[eernstg]

This would again rely on the ability for Dart code to see a difference between JS null and JS undefined. Similarly for line 27.

[eernstg]

Thanks, @srujzs!

@sgrekhov, do we use status files to enable a test for specific platforms?

[sgrekhov]

Yes, we use status files for these purposes. But in this particular case I think that it's simpler to use if(isJS) {/* Code, not supported by Wasm*/}

[eernstg]

it's simpler to use if(isJS) {/* Code, not supported by Wasm*/}

OK. I'm just wondering, if this test is executed on any platform where the generated code isn't JavaScript, presumably the eval thing wouldn't work. So how can we even use isJS if part of the test will be a compile-time error when isJS is false?

[srujzs]

if this test is executed on any platform where the generated code isn't JavaScript, presumably the eval thing wouldn't work

eval here is an interop method that interfaces https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval, so it should work even when compiling to Wasm. We use it often in the SDK tests for a quick way to set up the JS environment.

[sgrekhov]

After rereading the docs... @srujzs is using of isNull/isUndefined a compile-time error on Wasm?

[srujzs]

It's a runtime-error (you specifically get an UnimplementedError) when compiling to Wasm.

[sgrekhov]

Thank you. I thought that it throws on undefined values only. Ok, if so, then I'll switch off isNull and isUndefined tests in status files. @eernstg PTAL.