[cfe] Create _ConstructorDeclaration through ConstructorDeclaration

This decouples the create of _ConstructorDeclaration from fragments,
such that synthetic constructor declarations can also be created
together with the name space.

Change-Id: I1e384639ea0fd8672c34164038f068057e8fbd9e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436242
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

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

@@ -86,6 +86,8 @@ abstract class FactoryDeclaration {
 
   void resolveRedirectingFactory(
       {required SourceLibraryBuilder libraryBuilder});
+
+  bool get isRedirectingFactory;
 }
 
 class FactoryDeclarationImpl
@@ -138,6 +140,9 @@ class FactoryDeclarationImpl
   // Coverage-ignore(suite): Not run.
   Uri get fileUri => _fragment.fileUri;
 
+  @override
+  bool get isRedirectingFactory => _fragment.redirectionTarget != null;
+
   @override
   List<FormalParameterBuilder>? get formals => _fragment.formals;