function assertEslintrcConfig(linter) {
	const { configType } = internalSlotsMap.get(linter);

	if (configType === "flat") {
		throw new Error(
			"This method cannot be used with flat config. Add your entries directly into the config array.",
		);
	}
}
Throws an error when the given linter is in flat config mode.
@param {Linter} linter The linter to check.
@returns {void}
@throws {Error} If the linter is in flat config mode.