Exception accessing undefined array when requesting completions.

[DanielRosenwasser]

It's hard for me to repro simply, but start out with the following:

class C<T, U, V> {}

class D extends C</**/

Then start typing at /**/ to get:

[Error - 3:25:29 PM] 'completions' request failed with error.
Error processing request. Cannot read property 'forEach' of undefined
TypeError: Cannot read property 'forEach' of undefined
    at symbolsToArray (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:27940:20)
    at Object.getExportsOfModuleAsArray [as getExportsOfModule] (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:24293:20)
    at symbolCanbeReferencedAtTypeLocation (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62854:55)
    at C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62818:36
    at Object.filter (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:1438:25)
    at filterGlobalCompletion (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62804:27)
    at tryGetGlobalSymbols (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62800:27)
    at getCompletionData (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62692:22)
    at Object.getCompletionsAtPosition (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:62312:34)
    at Object.getCompletionsAtPosition (C:\Program Files\nodejs\node_modules\typescript\lib\tsserver.js:74328:35)

I can't reproduce this in a fourslash test or in vscode with typescript@next.
However, this is probably happening because completions calls getExportsOfModule on non-modules. This might be fixed by #16768. It would be nice to have a test case to prove it though.
Should getExportsOfModule just assert that the symbol is a module symbol?