[native_assets_cli] Fix syntax required fields

[dcharkes]

Closes #2039

Makes fields required in the syntax instead of semantic errors. The code for the semantic errors is simply deleted now.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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[mosuem]

This is a style question, and not something for the current PR, but I personally like setting nullable parameter as required to make people think about if they want to leave out the argument.

[dcharkes]

I do like that idea in this context. It means that the semantic layer around it explicitly has to pass null. And most likely if we're adding a new optional field, we should not be forgetting to pass it from the semantic API to the syntax.

Edit: err, this is the semantic API used by users. It would make file: null for all code assets with the link mode not requiring a file. I think that might be overly verbose.

[coveralls]

coverage: 80.542% (-5.4%) from 85.933%
when pulling a56ca2a on syntax-required-fields
into 4e534d3 on main.