[cross_file] Fix accidental constructor change

[stuartmorgan-g]

In #3095 I accidentally changed a constructor argument's nullability because I wasn't familiar with the correct variation of super parameters, and addressed a lint warning incorrectly. That version has been retracted; this restores the original behavior.

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• I added new tests to check the change I am making, or this PR is test-exempt.
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[stuartmorgan-g]

The test is convoluted, but it works (and is arguably no more convoluted than the three-files-that-must-match structure of the class itself).

[stuartmorgan-g]

Filed dart-lang/sdk#51196 for the false positive.

Although it's not actually clear to me why XFileBase has a constructor at all. Do you know what the reason was?

[asashour]

A humble suggestion, the code could be written as:

XFile(
Stringsuper.path, {
String? mimeType,
...
}) {

[stuartmorgan-g]

A humble suggestion, the code could be written as

Indeed; fixed, and updated the PR description to reflect that the analyzer wasn't at all the problem. Thanks!

[ditman]

LGTM! Thanks for the test!!

[ditman]

it's not actually clear to me why XFileBase has a constructor at all. Do you know what the reason was?

@stuartmorgan I don't remember, my only suspicion is to attempt to mandate that all subclasses (at least) have a path parameter? Not sure that's a valid solution (or requirement!)