Provide a compatible method to conditionally import between WASM and regular Web with also backward compatibility

[AlexV525]

Dio is embedded with the Web HTTP request support based on dart:html. It also widely supports Dart SDK >=2.18.0 applications.

During the release of the WASM support, the package:web requires Dart SDK >=3.3.0 which is way smaller than our supported range.

Many contributors including ourselves have tried to find a static way to split the Web support into packages without making any breaking changes, but that seems impossible because we need a compatible way to conditionally import libraries based on the environment, which is not provided by Dart if you applied the above restrictions.

• Adapt dart2wasm cfug/dio#2215
• Split the web support to package dio_browser_adapter cfug/dio#2218
• ⚡️ Split web_adapter cfug/dio#2223

Please consider supporting the request or kindly provide a workaround, because there are millions of applications that are using Dio. Huge thanks for any input.

[Rexios80]

What if you do something like:

import 'stub.dart'
  if (dart.library.html) 'html_impl.dart'
  if (dart.library.js_interop) 'js_interop_impl.dart';

If the evaludation is short circuited, then the html implementation should be used on old web platforms and the js_interop implementation should be used on WASM.

[AlexV525]

What if you do something like:

import 'stub.dart'
  if (dart.library.html) 'html_impl.dart'
  if (dart.library.js_interop) 'js_interop_impl.dart';

If the evaludation is short circuited, then the html implementation should be used on old web platforms and the js_interop implementation should be used on WASM.

This could work if the condition is not running into any compile issues.

[sigmundch]

Potentially we could treat this as a duplicate of #55694?

cc @osa1

[osa1]

Potentially we could treat this as a duplicate of #55694?

Yes, I think this is a duplicate #55694.

[sigmundch]

I'll close this so we can track the idea of the new condition in the other issue.

I forgot to mention - the workaround suggested by @Rexios80 above is also what we recommend for distinguishing runtimes under the current available conditional import conditions.

dart:html is supported in JavaScript compilers (DDC and dart2js) and not Wasm, so dart.library.html can be used to distinguish between the various JS runtimes, similarly dart.library.js_interop helps distinguish wasm from the VM runtime.

In the past some people have used js and js_util, but it's likely those will not be supported in Wasm in the future, and as such, we normally recommend using .js_interop instead.

This could work if the condition is not running into any compile issues.

Can you clarify what's your concern? I'm not sure I follow