Parts must have exactly the same language version override as the libraryDRT-W1394
The analyzer produces this diagnostic when a part file has a language version override comment that specifies a different language version than the one being used for the library to which the part belongs.
Example
Given a part file named part.dart
that contains the following:
// @dart = 2.14
part of 'test.dart';
The following code produces this diagnostic because the parts of a library must have the same language version as the defining compilation unit:
// @dart = 2.15
part 'part.dart';
Common fixes
Remove the language version override from the part file, so that it implicitly uses the same version as the defining compilation unit:
part of 'test.dart';
If necessary, either adjust the language version override in the defining compilation unit to be appropriate for the code in the part, or migrate the code in the part file to be consistent with the new language version.