Support for extension types in strict modes

[srawlins]

There may not be much to do here, but here are some examples:

• This should be reported for strict-raw-types:

  extension type ET(List it) {}

• This should be reported for strict-inference:

  extension type ET<T>(int it) {}
  
  var x = ET(it); // could not infer T!

• This for strict-casts:

  extension type ET(int it) {}
  
  dynamic d = "hello";
  var x = ET(d); // cast from dynamic!