In the semantics tree, do not detach a child if it has already been assigned a new parent (#6773)

Fixes https://github.com/flutter/flutter/issues/6690

Also add a version of the Gallery smoke test that enables semantics

examples/flutter_gallery/test/smoke_test.dart

@@ -5,9 +5,10 @@
 import 'dart:collection' show LinkedHashSet;
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter_gallery/gallery/item.dart' show GalleryItem, kAllGalleryItems;
-import 'package:flutter_gallery/main.dart' as flutter_gallery_main;
+import 'package:flutter_gallery/gallery/app.dart' show GalleryApp;
 
 const String kCaption = 'Flutter Gallery';
 
@@ -47,10 +48,8 @@ Future<Null> smokeDemo(WidgetTester tester, String routeName) async {
   return null;
 }
 
-void main() {
-  testWidgets('Flutter Gallery app smoke test', (WidgetTester tester) async {
-    flutter_gallery_main
-        .main(); // builds the app and schedules a frame but doesn't trigger one
+Future<Null> runSmokeTest(WidgetTester tester) async {
+  await tester.pumpWidget(new GalleryApp());
   await tester.pump(); // see https://github.com/flutter/flutter/issues/1865
   await tester.pump(); // triggers a frame
 
@@ -90,5 +89,14 @@ void main() {
   await tester.tap(find.text('Light'));
   await tester.pump();
   await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
+}
+
+void main() {
+  testWidgets('Flutter Gallery app smoke test', runSmokeTest);
+
+  testWidgets('Flutter Gallery app smoke test', (WidgetTester tester) async {
+    RendererBinding.instance.setSemanticsEnabled(true);
+    await runSmokeTest(tester);
+    RendererBinding.instance.setSemanticsEnabled(false);
   });
 }

packages/flutter/lib/src/rendering/binding.dart

@@ -138,7 +138,11 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   SemanticsHandle _semanticsHandle;
 
   void _handleSemanticsEnabledChanged() {
-    if (ui.window.semanticsEnabled) {
+    setSemanticsEnabled(ui.window.semanticsEnabled);
+  }
+
+  void setSemanticsEnabled(bool enabled) {
+    if (enabled) {
       _semanticsHandle ??= _pipelineOwner.ensureSemantics();
     } else {
       _semanticsHandle?.dispose();

packages/flutter/lib/src/rendering/semantics.dart

@@ -372,6 +372,7 @@ class SemanticsNode extends AbstractNode {
   /// child list for the given frame at once instead of needing to process the
   /// changes incrementally as new children are compiled.
   void finalizeChildren() {
+    // The goal of this function is updating sawChange.
     if (_children != null) {
       for (SemanticsNode child in _children)
         child._dead = true;
@@ -473,10 +474,14 @@ class SemanticsNode extends AbstractNode {
     owner._detachedNodes.add(this);
     super.detach();
     if (_children != null) {
-      for (SemanticsNode child in _children)
+      for (SemanticsNode child in _children) {
+        // The list of children may be stale and may contain nodes that have
+        // been assigned to a different parent.
+        if (child.parent == this)
           child.detach();
       }
     }
+  }
 
   bool _dirty = false;
   void _markDirty() {