Make mixin and interface a builtin identifier or reserved word

[leafpetersen]

We should reserve these now for future compatibility (i.e. with the upcoming mixin feature).

Is builtin identifier enough, or do we need this to be a reserve word?

cc @lrhn @eernstg

Update: Make them builtin identifiers.

• Analyzer issue: Analyzer: mixin and interface should be builtin identifiers in Dart 2 #33131
• Front end issue: Front end: mixin and interface should be builtin identifiers in Dart 2  #33132
• Spec issue: Spec: mixin and interface should be builtin identifiers in Dart 2 #33133

[lrhn]

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

[leafpetersen]

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> {}