[cfe] Implement synthetic constructors through ConstructorDeclaration

This implements the synthesized constructors for default constructors
and forwarding constructors, avoiding the need for
SyntheticSourceConstructorBuilder.

Change-Id: Iaf7234aab658122a29c644226bf82a63f7895246
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432700
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/fragment/constructor.dart

@@ -31,7 +31,7 @@ class ConstructorFragment implements Fragment, FunctionFragment {
   final DeclarationFragment enclosingDeclaration;
   final LibraryFragment enclosingCompilationUnit;
 
-  SourceConstructorBuilderImpl? _builder;
+  SourceConstructorBuilder? _builder;
 
   ConstructorFragmentDeclaration? _declaration;
 
@@ -68,12 +68,12 @@ class ConstructorFragment implements Fragment, FunctionFragment {
   }
 
   @override
-  SourceConstructorBuilderImpl get builder {
+  SourceConstructorBuilder get builder {
     assert(_builder != null, "Builder has not been computed for $this.");
     return _builder!;
   }
 
-  void set builder(SourceConstructorBuilderImpl value) {
+  void set builder(SourceConstructorBuilder value) {
     assert(_builder == null, "Builder has already been computed for $this.");
     _builder = value;
   }

pkg/front_end/lib/src/fragment/constructor/body_builder_context.dart

@@ -20,7 +20,7 @@ import '../../type_inference/type_inferrer.dart';
 import 'declaration.dart';
 
 class ConstructorBodyBuilderContext extends BodyBuilderContext {
-  final SourceConstructorBuilderImpl _builder;
+  final SourceConstructorBuilder _builder;
 
   final ConstructorFragmentDeclaration _declaration;