analyzer: Rename ErrorSeverityTest to DiagnosticSeverityTest

Work towards #60635

Change-Id: I1361a28834cb95a8b7ad9f131132b7b77e888483
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/430547
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

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

@@ -20,9 +20,6 @@ typedef ErrorCode = DiagnosticCode;
 ///
 /// Generally, messages should follow the [Guide for Writing
 /// Diagnostics](https://github.com/dart-lang/sdk/blob/main/pkg/front_end/lib/src/base/diagnostics.md).
-///
-/// Note that this class name, `ErrorCode`, is soft-deprecated in favor of the
-/// type alias, `DiagnosticCode`.
 @AnalyzerPublicApi(message: 'exported by package:analyzer/error/error.dart')
 abstract class DiagnosticCode {
   /// Regular expression for identifying positional arguments in error messages.
@@ -139,9 +136,6 @@ typedef ErrorSeverity = DiagnosticSeverity;
 
 /**
  * The severity of an [DiagnosticCode].
- *
- * Note that this class name, `ErrorSeverity`, is soft-deprecated in favor of
- * the type alias, [DiagnosticSeverity].
  */
 @AnalyzerPublicApi(message: 'exported by package:analyzer/error/error.dart')
 class DiagnosticSeverity implements Comparable<DiagnosticSeverity> {

pkg/analyzer/test/generated/all_the_rest_test.dart

@@ -15,7 +15,7 @@ import 'package:test_reflective_loader/test_reflective_loader.dart';
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(DartUriResolverTest);
-    defineReflectiveTests(ErrorSeverityTest);
+    defineReflectiveTests(DiagnosticSeverityTest);
     defineReflectiveTests(ResolveRelativeUriTest);
   });
 }
@@ -74,7 +74,7 @@ class DartUriResolverTest extends _SimpleDartSdkTest {
 }
 
 @reflectiveTest
-class ErrorSeverityTest {
+class DiagnosticSeverityTest {
   test_max_error_error() async {
     expect(
       DiagnosticSeverity.ERROR.max(DiagnosticSeverity.ERROR),