I believe being a built-in identifier is enough. The words are only expected at the beginning of a top-level declaration (likely not even preceded by abstract), so preventing them from being types or prefixes will allow us to recognize their meaning immediately.

Making them reserved words is a much more breaking change, since it would break existing code that uses the names as variables or class members. Such code exists.

@peter-ahe-google

Concretely (for reference) all of the following will become errors (and similarly for interface):

import "dart:core" as mixin;

typedef mixin(x);

class mixin {}

class Foo<mixin> {}