CFE fails to account for null when checking whether a switch statement as exhaustive

[stereotype441]

The following program is accepted by the CFE:

enum A {
  b, c
}

int f(A? a) {
  switch (a) {
    case A.b:
    case A.c:
    return 0;
  }
}

main() {
  f(A.b);
}

It should be rejected, since the switch statement is non-exhaustive (it doesn't handle null), and therefore null might be returned from f, in violation of its static type. This is a soundness issue.

[stereotype441]

See also dart-lang/language#1143 (comment)

[franklinyow]

@leafpetersen Can you triage this? Thx