asserts assertion guard not downleveled when used as type annotation

[spenceryue]

Input

export const foo: (_: number | string) => asserts _ is number = (_) => {};

Output

export declare const foo: (_: number | string) => asserts _ is number;

Workaround

export function foo(_: number | string): asserts _ is number {}

---

Details

node	12.18.2
downlevel-dts	0.6.0
command	node_modules/.bin/downlevel-dts dist/my-package dist/my-package/ts3.4 --to=3.4