[analyzer] Remove CompilationUnitMemberTest._updateBeginToken.

The sole purpose of this method was in parser recovery tests, to
adjust `CompilationUnit.beginToken` after parser recovery, to work
around the fact that a mismatch in begin tokens caused
`ResultComparator` to report a failure in parser recovery.

Now that `ResultComparator` doesn't examine
`CompilationUnit.beginToken` (see
https://dart-review.googlesource.com/c/sdk/+/419521), this is
unnecessary.

Removing this logic allows `CompilationUnit.beginToken` to be changed
to a final field.

Change-Id: Ic2528b5ff9dd9f1aec0fade4499b78ff165eff02
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419680
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -3476,7 +3476,7 @@ final class CompilationUnitImpl extends AstNodeImpl
     with AstNodeWithNameScopeMixin
     implements CompilationUnit {
   @override
-  Token beginToken;
+  final Token beginToken;
 
   ScriptTagImpl? _scriptTag;
 

pkg/analyzer/test/src/fasta/recovery/code_order_test.dart

@@ -115,7 +115,7 @@ export 'bar.dart';
 ''', [ParserErrorCode.DIRECTIVE_AFTER_DECLARATION], '''
 export 'bar.dart';
 class C { }
-''', adjustValidUnitBeforeComparison: _updateBeginToken);
+''');
   }
 
   void test_declarationBeforeDirective_import() {
@@ -125,7 +125,7 @@ import 'bar.dart';
 ''', [ParserErrorCode.DIRECTIVE_AFTER_DECLARATION], '''
 import 'bar.dart';
 class C { }
-''', adjustValidUnitBeforeComparison: _updateBeginToken);
+''');
   }
 
   void test_declarationBeforeDirective_part() {
@@ -135,7 +135,7 @@ part 'bar.dart';
 ''', [ParserErrorCode.DIRECTIVE_AFTER_DECLARATION], '''
 part 'bar.dart';
 class C { }
-''', adjustValidUnitBeforeComparison: _updateBeginToken);
+''');
   }
 
   void test_declarationBeforeDirective_part_of() {
@@ -145,7 +145,7 @@ part of foo;
 ''', [ParserErrorCode.DIRECTIVE_AFTER_DECLARATION], '''
 part of foo;
 class C { }
-''', adjustValidUnitBeforeComparison: _updateBeginToken);
+''');
   }
 
   void test_exportBeforeLibrary() {
@@ -194,11 +194,6 @@ part 'foo.dart';
       invalidNodes: [],
     );
   }
-
-  CompilationUnitImpl _updateBeginToken(CompilationUnitImpl unit) {
-    unit.beginToken = unit.declarations[0].beginToken;
-    return unit;
-  }
 }
 
 /// Test how well the parser recovers when the members of an import directive