[cross_file_web] Adds support to instantiate the web PlatformXFile without needing to import web apis - #13395

Code Review

This pull request adds support for creating a WebScopedStorageXFile from bytes by introducing the WebScopedStorageXFileCreationParams.fromBytes factory constructor, updating the changelog, and adding integration tests. The review feedback suggests correcting a grammatical typo in the new constructor's documentation and expanding test coverage to verify behavior when fileName is null.

Code Review

This pull request introduces support for creating a WebScopedStorageXFile from bytes on the web, along with a platform-safe entry point (cross_file_web_safe.dart) to prevent compilation failures on non-web platforms. It adds the WebScopedStorageXFileCreationParams.fromBytes constructor, corresponding stub implementations, integration tests, and updated documentation. Feedback on the changes suggests removing the const keyword from the fromBytes constructor in the safe stub to maintain compilation consistency with the non-const factory constructor in the actual web implementation.

The approach of a web-safe library with a conditional export looks good. Two points on the non-web stub, as line comments, plus some smaller notes:

• The stub's WebScopedStorageXFile class is not exported by cross_file_web_safe.dart, which exports only WebScopedStorageXFileCreationParams and WebScopedStorageXFileExtension, so roughly 60 lines of the stub look unreachable. Is it needed?
• No test exercises the stub's behaviour: the integration tests run on web and so go through the real implementation, and tests_exist_elsewhere_test.dart only checks that the import compiles on the VM.
• In the integration test, the import alias web_safe_file refers to src/web_scoped_storage_cross_file.dart, the web-only library, which reads as if it were the safe one.
• Minor: the stub has no fromObjectUrl, although that factory takes only a String; and testOverrides is typed dynamic. Both may be deliberate.

Not verified: CI status, and compiling the first line comment's case on both platforms; that point rests on the language rule that a sealed class cannot be instantiated.

Since it's an API addition it looks like maybe @stuartmorgan-g should also take a look?

+1 to all of @davidmorgan's comments above. Left a few additional non-overlapping notes below on exporting CrossFileWeb and fromObjectUrl from the safe library and how the download integration test can use the web_safe import directly.

I am also still new to this area and learning, let me know if I am off the mark on some of these points. :)

The updates address the earlier comments: the stub params class is now sealed, both stub factories throw UnsupportedError, the unreachable stub WebScopedStorageXFile is gone, the VM test exercises the stub, and the integration test imports the safe library. LGTM, with one CHANGELOG nit inline.