Added if-tests for #1904
Updated tests because of minor change to the way else-blocks are created (#1904)
Fixed some tests for #1904
Fixed #1904 Implement `narrow type in else block of if (is...)`
Handle casting of `Sequential<>` to `Empty` (#1904)
Make the `exists` and `nonempty` constructs support negation (#1904)
Fixed if-related tests for #1904
Added switch-test for #1904
Implemented narrowing `else` clauses for `switch` statments (#1904)
Fixed switch-related tests for #1904