avoid AnimatedBuilder rebuilding widget (#55832)

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -369,6 +369,22 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
 
     return AnimatedBuilder(
       animation: widget.route.animation,
+      child: BottomSheet(
+        animationController: widget.route._animationController,
+        onClosing: () {
+          if (widget.route.isCurrent) {
+            Navigator.pop(context);
+          }
+        },
+        builder: widget.route.builder,
+        backgroundColor: widget.backgroundColor,
+        elevation: widget.elevation,
+        shape: widget.shape,
+        clipBehavior: widget.clipBehavior,
+        enableDrag: widget.enableDrag,
+        onDragStart: handleDragStart,
+        onDragEnd: handleDragEnd,
+      ),
       builder: (BuildContext context, Widget child) {
         // Disable the initial animation when accessible navigation is on so
         // that the semantics are added to the tree at the correct time.
@@ -383,22 +399,7 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
           child: ClipRect(
             child: CustomSingleChildLayout(
               delegate: _ModalBottomSheetLayout(animationValue, widget.isScrollControlled),
-              child: BottomSheet(
-                animationController: widget.route._animationController,
-                onClosing: () {
-                  if (widget.route.isCurrent) {
-                    Navigator.pop(context);
-                  }
-                },
-                builder: widget.route.builder,
-                backgroundColor: widget.backgroundColor,
-                elevation: widget.elevation,
-                shape: widget.shape,
-                clipBehavior: widget.clipBehavior,
-                enableDrag: widget.enableDrag,
-                onDragStart: handleDragStart,
-                onDragEnd: handleDragEnd,
-              ),
+              child: child,
             ),
           ),
         );

packages/flutter/test/material/bottom_sheet_test.dart

@@ -306,6 +306,38 @@ void main() {
     expect(find.text('BottomSheet'), findsNothing);
   });
 
+  testWidgets('Modal BottomSheet builder should only be called once', (WidgetTester tester) async {
+    BuildContext savedContext;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Builder(
+        builder: (BuildContext context) {
+          savedContext = context;
+          return Container();
+        },
+      ),
+    ));
+
+    int numBuilderCalls = 0;
+    showModalBottomSheet<void>(
+      context: savedContext,
+      isDismissible: false,
+      enableDrag: true,
+      builder: (BuildContext context) {
+        numBuilderCalls++;
+        return const Text('BottomSheet');
+      },
+    );
+
+    await tester.pumpAndSettle();
+    expect(numBuilderCalls, 1);
+
+    // Swipe the bottom sheet to dismiss it.
+    await tester.drag(find.text('BottomSheet'), const Offset(0.0, 150.0));
+    await tester.pumpAndSettle(); // Bottom sheet dismiss animation.
+    expect(numBuilderCalls, 1);
+  });
+
   testWidgets('Verify that a downwards fling dismisses a persistent BottomSheet', (WidgetTester tester) async {
     final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
     bool showBottomSheetThenCalled = false;