Stop using deprecated ParserErrorCode.UNEXPECTED_TOKEN.

[stereotype441]

In https://dart-review.googlesource.com/c/sdk/+/444921, all analyzer error constants were renamed to camelCase, but
ParserErrorCode.UNEXPECTED_TOKEN was left in place (and deprecated) since it is used by dart_style.

This change updates dart_style to use the new camelCase declaration.

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[munificent]

Looks like you'll need to also need to update the CI script to use that version of analyzer too, here and here.

[stereotype441]

Looks like you'll need to also need to update the CI script to use that version of analyzer too, here and here.

Ok, thanks! Looks like I will have to wait for Dart 3.10 to be published. No problem: this work is not urgent. I'll put it on hold until then.