ts.sys.getModifiedTime has wrong return type

[AviVahl]

Tiny issue. ts.sys.getModifiedTime has the following signature:

getModifiedTime?(path: string): Date;

But it actually returns undefined for invalid paths.

TypeScript Version: 2.9.2

Search Terms:
getModifiedTime

Code

const modifiedTime = ts.sys.getModifiedTime(`/invalid/path`)

Expected behavior:
Signature should match implementation. In this case:

getModifiedTime?(path: string): Date | undefined;

Actual behavior:
Return type can be undefined
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