[cross_file] [web] Separate "Save As" implementation details from XFile web class

[raju-muliyashiya]

fixes #91400

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[gemini-code-assist]

Code Review

This pull request refactors the "Save As" functionality for the web implementation by moving it from the XFile class into a dedicated helper file. This is a positive change that improves separation of concerns and makes the XFile class cleaner. My review includes suggestions to enhance the new testing mechanism to prevent test pollution and a minor code simplification for better maintainability.

[gemini-code-assist]

Modifying a global variable (helpers.anchorElementOverride) in a test can lead to test pollution, where one test's state affects another. It's crucial to clean up this state after the test runs. You can use addTearDown to ensure the override is reset.

Additionally, the test may create a DOM element that should also be cleaned up to prevent side effects between tests.

        addTearDown(() {
          helpers.anchorElementOverride = null;
          html.document.querySelector('#$crossFileDomElementId')?.remove();
        });
        helpers.anchorElementOverride = (_, __) => mockAnchor;

        final XFile file = XFile.fromData(bytes, name: textFile.name);

[gemini-code-assist]

Using a global variable for testing can lead to test pollution. It's good practice to:

1. Annotate it with @visibleForTesting to signal its intended use. This requires importing package:meta/meta.dart at the top of the file.
2. Add a comment reminding developers to clean it up in their tests using tearDown or addTearDown.

/// Override for creating anchor elements for testing purposes.
///
/// This should be reset to `null` in a `tearDown` block to avoid test pollution.
@visibleForTesting
CreateAnchorElement? anchorElementOverride;

[gemini-code-assist]

Instead of a while loop to remove child elements, you can use the more modern and concise Element.replaceChildren() method to clear the container.

  target.replaceChildren();