Legacy mode nullability and exhaustiveness checking

[munificent]

Consider:

// In pre-null-safety code:
main() {
  print(Foo(null));
}

// In modern Dart with pattern support:
class Foo {
  final bool b;
  Foo(this.b);
}

String f(Foo f) {
  switch (f) {
    case Foo(b: true): return 'Foo(true)';
    case Foo(b: false): return 'Foo(false)';
  }
}

This switch isn't actually exhaustive if an instance of Foo can flow in from legacy mode code where b may actually be null. We need to specify how this is handled.

I think the right answer is to have the switch throw a runtime exception if no case matches. The only time this can happen is when legacy code interacts with null safe code. In a fully migrated program, switches can be soundly statically checked for exhaustiveness.

cc @stereotype441

[stereotype441]

Note: a similar question came up during the work on null safety (with regard to enum-based switch statements), and we decided that it should throw: #1143 (comment)

[munificent]

Closing in favor of #2178 (which is done).