[messages] Get rid of deprecatedSnakeCaseNames.

The only remaining deprecated snake-case name was
`ParserErrorCode.UNEXPECTED_TOKEN`, which is no longer being used by
`package:dart_style`.

Note that the commit that removed `package:dart_style`'s use of
`ParserErrorCode.UNEXPECTED_TOKEN`
(https://github.com/dart-lang/dart_style/commit/ade6076f731c39306e3dd32608dfd00622e4492f)
was also the commit that bumped `package:dart_style`'s pubspec
dependency to permit analyzer versions >= `9.0.0`. Since the current
version of `package:analyzer` is `9.0.1-dev`, this means that `pub
get` will never pair a future version of `dart:analyzer` with a
version of `package:dart_style` that uses
`ParserErrorCode.UNEXPECTED_TOKEN`, so this removal is safe.

Change-Id: I6a6a6964a6e1dde6d8c1f1420b39f998b8a4073a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/462322
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/error/syntactic_errors.g.dart

@@ -1116,9 +1116,6 @@ class ParserErrorCode {
   static const DiagnosticWithoutArguments typeParameterOnOperator =
       diag.typeParameterOnOperator;
 
-  @Deprecated("Please use unexpectedToken")
-  static const DiagnosticCode UNEXPECTED_TOKEN = unexpectedToken;
-
   /// Parameters:
   /// Object p0: the starting character that was missing
   static const DiagnosticWithArguments<

pkg/analyzer/lib/src/diagnostic/diagnostic.g.dart

@@ -16094,9 +16094,6 @@ undone = DiagnosticWithArguments(
   expectedTypes: [ExpectedType.string],
 );
 
-@Deprecated("Please use unexpectedToken")
-const DiagnosticCode UNEXPECTED_TOKEN = unexpectedToken;
-
 /// No parameters.
 const DiagnosticWithoutArguments
 unexpectedDollarInString = DiagnosticWithoutArgumentsImpl(

pkg/analyzer_utilities/lib/analyzer_messages.dart

@@ -81,9 +81,6 @@ const List<DiagnosticClassInfo> diagnosticClasses = [
     file: syntacticErrorsFile,
     name: 'ParserErrorCode',
     type: AnalyzerDiagnosticType.syntacticError,
-    deprecatedSnakeCaseNames: {
-      'UNEXPECTED_TOKEN', // Referenced by `package:dart_style`.
-    },
   ),
   DiagnosticClassInfo(
     file: manifestWarningCodeFile,
@@ -584,11 +581,6 @@ class DiagnosticClassInfo {
   /// The type of diagnostics in this class.
   final AnalyzerDiagnosticType type;
 
-  /// The names of any diagnostics which are relied upon by analyzer clients,
-  /// and therefore will need their "snake case" form preserved (with a
-  /// deprecation notice) after migration to camel case diagnostic codes.
-  final Set<String> deprecatedSnakeCaseNames;
-
   /// Documentation comment to generate for the diagnostic class.
   ///
   /// If no documentation comment is needed, this should be the empty string.
@@ -598,7 +590,6 @@ class DiagnosticClassInfo {
     required this.file,
     required this.name,
     required this.type,
-    this.deprecatedSnakeCaseNames = const {},
     this.comment = '',
   });
 
@@ -742,14 +733,6 @@ LocatableDiagnostic $withArgumentsName({$withArgumentsParams}) {
     constant.writeln('expectedTypes: ${_computeExpectedTypes()},');
     constant.writeln(');');
     memberAccumulator.constants[constantName] = constant.toString();
-
-    if (diagnosticClassInfo.deprecatedSnakeCaseNames.contains(diagnosticCode)) {
-      memberAccumulator.constants[diagnosticCode] =
-          '''
-@Deprecated("Please use $constantName")
-const DiagnosticCode $diagnosticCode = $constantName;
-''';
-    }
   }
 
   /// Generates doc comments for this error code.
@@ -799,21 +782,12 @@ const DiagnosticCode $diagnosticCode = $constantName;
   /// diagnostic, imported from `diagnostic.g.dart` using the import prefix
   /// `diag`.
   void toAnalyzerRedirectCode({required MemberAccumulator memberAccumulator}) {
-    var diagnosticCode = analyzerCode.snakeCaseName;
     var ConstantStyle(:staticType) = constantStyle;
     var constant = StringBuffer();
     outputConstantHeader(constant);
     constant.writeln('  static const $staticType $constantName =');
     constant.writeln('    diag.$constantName;');
     memberAccumulator.constants[constantName] = constant.toString();
-
-    if (diagnosticClassInfo.deprecatedSnakeCaseNames.contains(diagnosticCode)) {
-      memberAccumulator.constants[diagnosticCode] =
-          '''
-  @Deprecated("Please use $constantName")
-  static const DiagnosticCode $diagnosticCode = $constantName;
-''';
-    }
   }
 
   /// Generates the appropriate declaration for this diagnostic to include in