Revert "Make WidgetPlaceholder.firstChild public"

This reverts commit 044c1a8.

Author: daohoangson

packages/core/lib/src/core_helpers.dart

@@ -95,10 +95,8 @@ class WidgetPlaceholder extends StatelessWidget {
   /// A human-readable description of this placeholder.
   final String? debugLabel;
 
-  /// The first child widget.
-  final Widget? firstChild;
-
   final List<WidgetPlaceholderBuilder> _builders;
+  final Widget? _firstChild;
 
   /// Creates a placeholder.
   WidgetPlaceholder({
@@ -107,13 +105,13 @@ class WidgetPlaceholder extends StatelessWidget {
     this.debugLabel,
     super.key,
   })  : _builders = builder != null ? [builder] : [],
-        firstChild = child;
+        _firstChild = child;
 
   /// Whether this placeholder renders anything.
-  bool get isEmpty => firstChild == null && _builders.isEmpty;
+  bool get isEmpty => _firstChild == null && _builders.isEmpty;
 
   @override
-  Widget build(BuildContext context) => callBuilders(context, firstChild);
+  Widget build(BuildContext context) => callBuilders(context, _firstChild);
 
   /// Calls builder callbacks on the specified [child] widget.
   @protected

packages/core/lib/src/internal/core_build_tree.dart

@@ -396,9 +396,6 @@ class _WidgetPlaceholderDefault extends StatelessWidget
   @override
   String? get debugLabel => null;
 
-  @override
-  Widget? get firstChild => null;
-
   @override
   bool get isEmpty => true;