[messages] Make some internal classes abstract.

Makes the classes `DiagnosticCodeImpl` and `LintCodeWithExpectedTypes`
abstract. These classes are never concretely instantiated (only
derived classes are), and they are private to the analyzer and linter
so they should not be concretely instantiated by clients either.

Making these classes abstract makes the class hierarchy for
diagnostics easier to reason about.

Change-Id: I6a6a696489d5a33dabcba48988a8473f0e819a85
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460860
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/base/errors.dart

@@ -317,7 +317,7 @@ abstract class DiagnosticCode {
   String toString() => uniqueName;
 }
 
-class DiagnosticCodeImpl extends DiagnosticCode {
+abstract class DiagnosticCodeImpl extends DiagnosticCode {
   @override
   final DiagnosticType type;
 

pkg/analyzer/lib/src/dart/error/lint_codes.dart

@@ -55,7 +55,7 @@ class LintCode extends DiagnosticCode {
 
 /// Private subtype of [LintCode] that supports runtime checking of parameter
 /// types.
-class LintCodeWithExpectedTypes extends DiagnosticCodeWithExpectedTypes
+abstract class LintCodeWithExpectedTypes extends DiagnosticCodeWithExpectedTypes
     implements LintCode {
   const LintCodeWithExpectedTypes({
     required super.name,