anchor.diff:    visitInvalidInitializer(InvalidInitializer node) {
       problem(node, "Invalid initializer.");
     }
  +
  +  @override
  +  visitUnknownType(UnknownType node) {
  +    // Note: we can't pass [node] to [problem] because it's not a [TreeNode].
  +    problem(null, "Unexpected appearance of the unknown type.");
  +  }
   }
  
positive.code:   @override
    problem(TreeNode node, String details) {
      VerificationError error = new VerificationError(context, node, details);
      printUnexpected(Uri.parse(fileUri), node.fileOffset, "$error");
      errors.add(error);
    }
  
  
negative.code:   return verifier.errors;
  }
  
  class FastaVerifyingVisitor extends VerifyingVisitor
      implements TypeSchemaVisitor {
    final List<VerificationError> errors = <VerificationError>[];
  
    String fileUri;
  