fix(core,web): compile isA checks on Dart < 3.12

[SelaseKay]

Summary

• Fixes [core]: firebase_core_web 3.11.0 fails to compile for web — isA<JSObject>() called on Object-typed catch variable #18611: firebase_core_web 3.11.0 fails to dart2js on Flutter < 3.44 (The method 'isA' isn't defined for the type 'Object').
• Keeps the [firebase_core_web]: FirebaseCoreWeb.app() masks the real error with an unsafe as JSError cast (same pattern as #18176, not covered by that fix) #18548 behavior: Dart exceptions from guardNotInitialized() (e.g. core/not-initialized) are rethrown instead of being force-cast to JSError.
• Does not raise the SDK constraint (sdk: ^3.6.0).

Why is! JSError instead of isA

#18552 added e.isA<JSObject>() on catch (e) so we could distinguish JS Firebase errors from Dart exceptions without an invalid_runtime_check_with_js_interop_types ignore.

That only compiles on Dart ≥ 3.12. isA lived on JSAny? until 3.12, when it moved to NullableObjectUtilExtension on Object? (Dart changelog). catch (e) types e as Object, so on Dart 3.6–3.11 the extension does not apply.

FlutterFire CI did not catch this: web jobs run on floating stable (currently Flutter 3.44 / Dart 3.12). Users still on Flutter 3.35 / 3.38 (Dart 3.9 / 3.10) — which sdk: ^3.6.0 still allows — cannot compile for web.

Raising the constraint to ^3.12.0 would “fix” the compile error by refusing to resolve on those SDKs. That would drop Flutter 3.35/3.38, which is a much larger break than this patch.

So this PR uses e is! JSError with the same lint ignore already used in FlutterFire web (firebase_auth_web, cloud_functions_web, and this file’s flutterfire_ignore_scripts path). That:

• compiles on Dart 3.6+
• still rethrows non-JS errors (the [firebase_core_web]: FirebaseCoreWeb.app() masks the real error with an unsafe as JSError cast (same pattern as #18176, not covered by that fix) #18548 fix)
• still maps JS FirebaseErrors through _getJSErrorCode / _catchJSError

We do not use (e as JSAny).isA<JSObject>() to keep calling isA. That recasts Dart exceptions into JS types and can reintroduce the #18548 TypeError, especially on dart2wasm.

Test plan

• flutter test --platform chrome in packages/firebase_core/firebase_core_web (covers the [firebase_core_web]: FirebaseCoreWeb.app() masks the real error with an unsafe as JSError cast (same pattern as #18176, not covered by that fix) #18548 core/not-initialized regression test)
• flutter build web against this package on Flutter 3.35 or 3.38 (the reported failure)
• flutter build web on current stable (3.44+) still succeeds

[gemini-code-assist]

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize the Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counterproductive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for GitHub and other Google products, sign up here.