Should constant matchers be value matchers

See pattern proposal.

The constant matcher can be used to match specific values. It must be a constant value. This makes sense for when it's used in a switch case matcher where we currently require expressions to be constant, but not necessarily in a more dynamic check like e is matcher.

Example:

Iterable<List<T>> listsWhichStartWith<T>(T value, Iterable<List<T>> lists) sync* {
  for (var p in lists) if (p is [value, ... var rest]) yield rest;  
}

(assuming the List rest-matcher TODO)

If we only allow constant values to be used as matchers, we won't be able to do parameterized pattern matching on provided values. I think we should allow that. Not necessarily in switch, but in is and as checks.

Start with the linked patterns feature specification, especially the constant-matcher section, and compare it with the issue's parameterized matching example. Determine how constant and value matchers should behave in switch cases versus is and as checks; done means the language specification has a clear decision covering those contexts.