build_web_compilers does not respect Dart language version from pubspec.yaml

[greglittlefield-wf]

In packages that use a language version lower than the current SDK, via SDK constraints in pubspec.yaml, I've noticed compilation errors that seem to indicate that they're instead compiled with a newer language version.

• Dart versions I tried, which reproduced the issue: 3.10.3, 3.9.0
• build_web_compilers versions affected: >4.4.0 (more info at the bottom of the issue)

For example, given the following reduced test case that has a package language version of 2.19:

• pubspec.yaml

  name: language_version_repro
  environment:
    # Lower bound sets a language version of 2.19
    sdk: '>=2.19.0 <4.0.0'
  dev_dependencies:
    build_web_compilers: ^4.4.5

• web/needs_less_than_3.0.dart

  class MixinClass  {
    foo() => 'foo';
  }
  
  // 3.0 no longer allows mixing in classes: https://dart.dev/resources/language/evolution#dart-3-0
  class B extends Object with MixinClass {}
  
  main() => print(B().foo());

• web/needs_less_than_3.7.dart

  // 3.7 no longer allows referencing `_`: https://dart.dev/resources/language/evolution#dart-3-7
  void function(dynamic _) => print(_);
  
  void main() => function('foo');

The build fails with the following errors:

$ dart run build_runner build web
…
log output for build_web_compilers:ddc (lazy) on web/needs_less_than_3.0.ddc.module                                                              
E Error compiling dartdevc module:language_version_repro|web/needs_less_than_3.0.ddc.js                                                          
  web/needs_less_than_3.0.dart:5:29: Error: The class 'MixinClass' can't be used as a mixin because it isn't a mixin class nor a mixin.          
  class B extends Object with MixinClass {}                                                                                                      
                              ^               
…
log output for build_web_compilers:ddc (lazy) on web/needs_less_than_3.7.ddc.module                                                              
E Error compiling dartdevc module:language_version_repro|web/needs_less_than_3.7.ddc.js                                                          
  web/needs_less_than_3.7.dart:1:35: Error: Undefined name '_'.                                                                                  
  void function(dynamic _) => print(_);                                                                                                          
                                    ^     
…

and it also fails with similar errors when compiling in dart2js, via --release.

Adding explicit language comments to each file (e.g., //@dart=2.19 and //@dart=3.6 respectively) works around the issue. So, it seems that those language versions are respected, but the one from pubspec.yaml is not.

I noticed that this issue did not occur in earlier versions of build_web_compilers. With these dependencies, the issue does not reproduce:

dev_dependencies:
  build_web_compilers: 4.3.2
  build_modules: 5.0.18 # need to pin this to avoid compile errors

And in the next versions of build_web_compilers/build_modules, the issue reproduces (as also reproduces in the latest version of build_web_compilers, as shown in the reduced test case above):

dev_dependencies:
  build_web_compilers: 4.4.0
  build_modules: 5.1.0 # need to pin this to avoid compile errors