In some cases TypeScript is unable to identify a valid
symbol for an export. In this case it returns an "unknown"
symbol, which does not reference any declarations.
This fix ensures that ngcc does not crash if such a symbol
is encountered by checking whether `symbol.declarations`
exists before accessing it.
The commit does not contain a unit test as it was not possible
to recreate a scenario that had such an "unknown" symbol in
the unit test environment. The fix has been manually checked
against that original issue; and also this check is equivalent to
similar checks elsewhere in the code, e.g.
https://github.com/angular/angular/blob/8d0de89e/packages/compiler-cli/src/ngtsc/reflection/src/typescript.ts#L309Fixes#34560
PR Close#34658
Pete Bacon Darwin
570574df5b