export_legacy_symbol

The symbol '{0}' is defined in a legacy library, and can't be re-exported from a library with null safety enabled.

The analyzer produces this diagnostic when a library that was opted in to null safety exports another library, and the exported library is opted out of null safety.

Given a library that is opted out of null safety:

// @dart = 2.8
String s;

The following code produces this diagnostic because it's exporting symbols from an opted-out library:

export 'optedOut.dart';

class C {}

If you're able to do so, migrate the exported library so that it doesn't need to opt out:

String? s;

If you can't migrate the library, then remove the export:

class C {}

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