Language Service typing for Diagnostic does not define file field as optional

[tomv564]

TypeScript Version: 2.2.1 / nightly (2.2.0-dev.201xxxxx)

Code

// in a project with compiler diagnostics:
const brokenFiles = service.getPreEmitDiagnostics().map(diag => diag.file.fileName);

Expected behavior:

The file field always contains a SourceFile as the Diagnostic type specifies,
or the Diagnostic type should mark the file field as optional.

Actual behavior:

file is missing in the compiler diagnostics sent back (and probably cannot be provided).

The origin of these 'incomplete' diagnostics is likely https://github.com/Microsoft/TypeScript/blob/2150a7796b0dfe83c5286755410e0fd1646ed86c/src/compiler/core.ts#L1235-L1251

[tomv564]

Sorry, duplicate of #15666