[analyzer] Add ClassMemberImpl.augmentKeyword.

All classes derived from `ClassMemberImpl` already had an
implementation of `augmentKeyword` except for
`PrimaryConstructorBodyImpl` (which was added as part of this CL).

In a follow-up CL, this will be used as part of a rewrite of
`MemberDuplicateDefinitionVerifier`, to allow members marked `augment`
to be excluded from duplicate checking.

Change-Id: I6a6a6964b8e41e3e5fde3e17428204d1e0898bbf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/468882
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

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

@@ -3305,6 +3305,9 @@ sealed class ClassMemberImpl extends DeclarationImpl implements ClassMember {
   /// Either or both of the [comment] and [metadata] can be `null` if the member
   /// doesn't have the corresponding attribute.
   ClassMemberImpl({required super.comment, required super.metadata});
+
+  /// The `augment` keyword, or `null` if the keyword was absent.
+  Token? get augmentKeyword;
 }
 
 /// The name of a class, enum, or extension type declaration.
@@ -19635,6 +19638,9 @@ final class PrimaryConstructorBodyImpl extends ClassMemberImpl
     _becomeParentOf(body);
   }
 
+  @override
+  Token? get augmentKeyword => null;
+
   @generated
   @override
   FunctionBodyImpl get body => _body;