Breaking change #53883: Disambiguate `extension type` to declare an extension type

[Erik Ernst]

This breaking change is a grammar modification that disambiguates certain declarations starting with `extension type`: They can be `extension` declarations today, but all declarations starting with `extension type` should be `extension type` declarations, for readability reasons.

Breaking change request: https://github.com/dart-lang/sdk/issues/53883.

Please comment on the issue if you expect this change to cause difficulties for you.

  best regards,

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Erik Ernst  -  Google Danmark ApS

Skt Petri Passage 5, 2 sal, 1165 København K, Denmark

CVR no. 28866984