updates for the latest version of extension types

[devoncarew]

• updates for the latest version of extension types

This addresses a recent analysis issue - lib/node/process.dart:17:21 - Extension types can't declare instance fields. Try replacing the field with a getter. - extension_type_declares_instance_field.

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[srujzs]

I'm okay with this (especially since we don't use it a setter), but note that this an error in the analyzer that I filed an issue for: dart-lang/sdk#53226.

[devoncarew]

Ah! I would like to land as that'll get the CI green, but it sounds like this is a workaround for an analyzer / extension types issue.