[dart2js] List.unmodifiable fails to run on web

[sunarya-thito]

Steps to Reproduce

1. Add this code

for (int i = 0; i < List.unmodifiable([]).length; i++) {}

for example

void main() {
  for (int i = 0; i < List.unmodifiable([]).length; i++) {}
  runApp(const MyApp());
}

2. Run it with flutter web (on profile or release mode)

flutter run -d chrome --release

or you can also compile it with dart2js, which generate the js code below

    main() {
      var t1, i, result;
      result.$flags = 3;
      for (t1 = type$.dynamic, i = 0; result = A.List_List$from([], false, t1), i < result.length; ++i)
        ;
    }

3. You will get this error

Uncaught Error: Error: Cannot set properties of undefined (setting '$flags')
    at main.dart:114:65
    at _wrapJsFunctionForAsync_closure.$protected (async_patch.dart:311:19)
    at _wrapJsFunctionForAsync_closure.call$2 (async_patch.dart:336:23)
    at _awaitOnObject_closure.call$1 (async_patch.dart:287:19)
    at _RootZone.runUnary$2$2 (zone.dart:1676:46)
    at _RootZone.runUnary$2 (zone.dart:1675:5)
    at _Future__propagateToListeners_handleValueCallback.call$0 (future_impl.dart:169:29)
    at Object._Future__propagateToListeners (future_impl.dart:931:13)
    at _Future._completeWithValue$1 (future_impl.dart:707:5)
    at _Future__asyncCompleteWithValue_closure.call$0 (future_impl.dart:777:7)

from

    main() {
      var t1, i, result;
      result.$flags = 3;
      for (t1 = type$.dynamic, i = 0; result = A.List_List$from([], false, t1), i < result.length; ++i)
        ;
    }

the result variable is not initialized with a value, but it tries to set its flag to 3

Dart Version

Dart SDK version: 3.6.0 (stable) (Thu Dec 5 07:46:24 2024 -0800) on "windows_x64"

I understand that the code isn't good or anything, but this shouldn't happen, right? I don't know what else it might cause, it runs perfectly on debug mode and other platform

[rakudrama]

Thanks for filing this bug and producing a nice repro. Definitely a bug.

The bug probably happens when List.unmodifiable was inlined into the for-loop condition.
A work-around would be based on preventing the inlining from happening.
This could be done in the SDK by putting @pragma('dart2js:never-inline') on the implementation of List.unmodifiable.
Perhaps the application has a method that returns List.unmodifiable and the pragma could be placed there.
That does not fix the bug, but prevents the result.$flags assignment from being inlined into the condition.

The assignment result.$flags = 3 should happen after the assignment to result.
It is very strange that this has been hoisted out of the loop.
I suspect that the bug is in some part of the 'codegen' phase after SSA optimizations, perhaps the part that tries to generate JavaScript for-loops from the SSA CFG.

[rakudrama]

The fix will be in the next stable release (February)