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I don't think that this is needed, or at least I don't understand why it's needed.

The BABEL_8_BREAKING (+ STRIP_BABEL_8_FLAG) only affects the compiled files and not the source files. We run linting and type-checking on the source files, which are not affected by those two flags.

However, when linting we use @babel/eslint-parser and some ESLint plugins directly from our repo: even if the source files are not affected by BABEL_8_BREAKING, it's the linter itself that changes behavior between Babel 7 and Babel 8. For this reason, running make lint in Babel 8 tests is valuable because the ESLint behaviour might change.

This reasoning doesn't affect type checking: the type checker is the same when using Babel 7 and when using Babel 8, so (given that the src files are the same) it should produce the same result.

The BABEL_8_BREAKING flag does affect sources. For example we have removed t.Noop in Babel 8 so path.isNoop is not defined in packages/babel-traverse/src/path/generated/validators.ts. If path.isNoop() is used somewhere, it will pass in Babel 7 but fail in Babel 8.