We don't know which value we will get, so it is fair to say that the value is unknown.

The implementation is relatively straightforward.

But it breaks a couple of language/ tests, e.g. language/type/constants_test/07, which can be reduced to:

const bool fromEnvironment =
    const bool.fromEnvironment("foo", defaultValue: true);

void testSwitch(args) {
  switch (args) {
    case fromEnvironment ? const MyType() : Type: //# 07: compile-time error
  }
}

class MyType implements Type {
  const MyType();
  bool operator ==(Object other) => identical(this, other);
}

We stopped reporting the compile-time error, because the result of evaluating fromEnvironment ? const MyType() : Type is unknown, which does not implement operator ==.