[messages] Move ScannerErrorCode into analyzer.

Moves the class `ScannerErrorCode`, from `package:_fe_analyzer_shared`
to `package:analyzer`.

The only use of this code in `package:front_end` is in tests, and it's
ok for `package:front_end` tests to refer to `package:analyzer`, so it
didn't need to be shared. Moving it into `package:analyzer` allows for
a cleaner separation between analyzer-specific code and code that is
truly shared between the analyzer and the CFE.

Change-Id: I6a6a69643800d664651cffda3a78059a9bb7756d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/455263
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/messages/codes.dart

@@ -2,7 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-/// @docImport 'package:_fe_analyzer_shared/src/scanner/errors.dart';
+/// @docImport 'package:analyzer/src/dart/error/syntactic_errors.dart';
 /// @docImport 'package:analyzer/src/fasta/ast_builder.dart';
 /// @docImport 'package:analyzer/src/fasta/error_converter.dart';
 /// @docImport 'package:analyzer/src/diagnostic/diagnostic_code_values.dart';

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

@@ -4,7 +4,4 @@
 
 import 'package:_fe_analyzer_shared/src/base/errors.dart';
 
-export 'package:_fe_analyzer_shared/src/scanner/errors.dart'
-    show ScannerErrorCode;
-
 part 'package:analyzer/src/dart/error/syntactic_errors.g.dart';