Change to conditional imports broke web platform

[daniel-v]

@jakemac53 's changes in #3170 as a response to @mraleph 's issue of dart-lang/sdk#46918 has had some side effects.

Namely, I cannot build any of my projects that depend on conditional imports to differentiate between VM and web platforms.
A good example would be package:universal_html that is publicly available.

• Dart SDK version: 2.13.4 (stable) (Wed Jun 23 13:08:41 2021 +0200) on "linux_x64"
• Affected builder package: build_modules v4.0.2 (v4.0.1 works fine)

Repro code

import 'package:universal_html/html.dart';

void main() {
  print(document.body != null);
}

Observed errors

With DDC:

# dart run build_runner serve web:8080
[SEVERE] build_modules:module_library on package:universal_html/js.dart: Invalid argument(s): Unsupported conditional import of `dart:js` found in universal_html|lib/js.dart:

export 'dart:js';

This environment does not support direct conditional imports of `dart:` libraries. Instead you must create a separate library which unconditionally imports (or exports) the `dart:` library that you want to use, and conditionally import (or export) that library.

With dart2js

# dart run build_runner serve -r --define 'build_web_compilers|entrypoint=compiler=dart2js' --define 'build_web_compilers|entrypoint=dart2js_args=["-O1"]' web:8080

[SEVERE] build_modules:module_library on package:universal_html/js.dart (cached):

Invalid argument(s): Unsupported conditional import of `dart:js` found in universal_html|lib/js.dart:

export 'dart:js';

[jakemac53]

The example you have shown was never intended to work. I think there was a bug where we allowed conditional imports/exports of dart: libraries if the default import was a dart: library.

I would forward fix the package to export a library that exports dart:js.

[jakemac53]

Note that if you attempted to compile for the vm (with build_vm_compilers) I am quite sure it would not have worked.