[analyzer] Fix output generation upon test failure to use camelCase.

When `GatheringDiagnosticListener.assertErrors` detects a test
failure, it outputs a suggestion for how to change the test
expectations to match the current behavior.

Since error code constants are in `camelCase` now, but
`DiagnosticCode.uniqueName` still takes the form
`ClassName.SCREAMING_CAPS` or `ClassName.snake_case`, this logic needs
to be updated to convert the text after the `.` to `camelCase`.

Change-Id: I6a6a696478e0443aab5184ac3331af9aac29a9aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445407
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/test/generated/test_support.dart

@@ -12,6 +12,7 @@ import 'package:analyzer/instrumentation/instrumentation.dart';
 import 'package:analyzer/source/line_info.dart';
 import 'package:analyzer/source/source.dart';
 import 'package:analyzer/src/generated/engine.dart';
+import 'package:analyzer/src/utilities/extensions/string.dart';
 import 'package:test/test.dart';
 
 /// A description of a message that is expected to be reported with an error.
@@ -271,7 +272,7 @@ class GatheringDiagnosticListener implements DiagnosticListener {
         for (Diagnostic actual in diagnostics) {
           List<DiagnosticMessage> contextMessages = actual.contextMessages;
           buffer.write('  error(');
-          buffer.write(actual.diagnosticCode);
+          buffer.write(actual.diagnosticCode.constantName);
           buffer.write(', ');
           buffer.write(actual.offset);
           buffer.write(', ');
@@ -554,3 +555,19 @@ class TestSourceWithUri extends TestSource {
     return false;
   }
 }
+
+extension on DiagnosticCode {
+  /// The name of the constant in the analyzer package (or other related
+  /// package) that represents this diagnostic code.
+  ///
+  /// This string is used when generating test failure messages that suggest how
+  /// to change test expectations to match the current behavior.
+  ///
+  /// For example, if the unique name is `TestClass.MY_ERROR`, this method will
+  /// return `TestClass.myError`.
+  String get constantName => switch (uniqueName.split('.')) {
+    [var className, var snakeCaseName] =>
+      '$className.${snakeCaseName.toCamelCase()}',
+    _ => throw StateError('Malformed DiagnosticCode: $uniqueName'),
+  };
+}