* Make custom `Utf8Decoder` replacement not extend platform class.

The Dart 3.0 class modifiers will make `Utf8Decoder`, and other
pure implementation classes, be `final`.

This replacement class does not need to extend the original
class, just like the `Utf8Codec` in the same file doesn't
extend the `Ut8Codec` from the platform libraries.

Instead it can just forward to a single `const` instance of the
original class.

* Forgot one constant

* Fix merge conflict.

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Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>