[cfe] Create declarations directly in _PropertyPreBuilder

This is done in preparation for including synthetic declarations in
the same SourcePropertyBuilder is the corresponding user defined aspect.

Change-Id: I26928301d710682dd2301122df159a3a941c0e6a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/431502
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/fragment/getter/declaration.dart

@@ -34,7 +34,7 @@ import 'encoding.dart';
 abstract class GetterDeclaration {
   Uri get fileUri;
 
-  UriOffsetLength? get uriOffset;
+  UriOffsetLength get uriOffset;
 
   GetterQuality get getterQuality;
 

pkg/front_end/lib/src/fragment/setter/declaration.dart

@@ -34,7 +34,7 @@ import 'encoding.dart';
 abstract class SetterDeclaration {
   Uri get fileUri;
 
-  UriOffsetLength? get uriOffset;
+  UriOffsetLength get uriOffset;
 
   List<MetadataBuilder>? get metadata;