[LayoutBuilder] Implements baseline logic to pass baseline to child (#65000)

packages/flutter/lib/src/widgets/layout_builder.dart

@@ -361,6 +361,13 @@ class _RenderLayoutBuilder extends RenderBox with RenderObjectWithChildMixin<Ren
     }
   }
 
+  @override
+  double computeDistanceToActualBaseline(TextBaseline baseline) {
+    if (child != null)
+      return child.getDistanceToActualBaseline(baseline);
+    return super.computeDistanceToActualBaseline(baseline);
+  }
+
   @override
   bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
     return child?.hitTest(result, position: position) ?? false;

packages/flutter/test/widgets/baseline_test.dart

@@ -93,6 +93,26 @@ void main() {
     await tester.pump();
     expect(calls, 2);
   });
+
+  testWidgets('LayoutBuilder returns child\'s baseline', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: Baseline(
+            baseline: 180.0,
+            baselineType: TextBaseline.alphabetic,
+            child: LayoutBuilder(
+              builder: (BuildContext context, BoxConstraints constraints) {
+                return BaselineDetector(() {});
+              },
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getRect(find.byType(BaselineDetector)).top, 160.0);
+  });
 }
 
 class BaselineDetector extends LeafRenderObjectWidget {
@@ -133,7 +153,7 @@ class RenderBaselineDetector extends RenderBox {
   double computeDistanceToActualBaseline(TextBaseline baseline) {
     if (callback != null)
       callback();
-    return 0.0;
+    return 20.0;
   }
 
   void dirty() {