Fix attaching a dirty, detached SemanticsNode (#4855)

Previously we triggered an assert.

Fixes #4850

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

@@ -362,7 +362,6 @@ class SemanticsNode extends AbstractNode {
       return;
     _dirty = true;
     if (attached) {
-      assert(!owner._dirtyNodes.contains(this));
       assert(!owner._detachedNodes.contains(this));
       owner._dirtyNodes.add(this);
     }
@@ -478,7 +477,7 @@ class SemanticsOwner {
 
   final VoidCallback _onLastListenerRemoved;
 
-  final List<SemanticsNode> _dirtyNodes = <SemanticsNode>[];
+  final Set<SemanticsNode> _dirtyNodes = new Set<SemanticsNode>();
   final Map<int, SemanticsNode> _nodes = <int, SemanticsNode>{};
   final Set<SemanticsNode> _detachedNodes = new Set<SemanticsNode>();
 
@@ -526,10 +525,13 @@ class SemanticsOwner {
     _detachedNodes.clear();
     if (_dirtyNodes.isEmpty)
       return;
-    _dirtyNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
-    for (int index = 0; index < _dirtyNodes.length; index += 1) {
-      // we mutate the list as we walk it here, which is why we use an index instead of an iterator
-      SemanticsNode node = _dirtyNodes[index];
+    List<SemanticsNode> visitedNodes = <SemanticsNode>[];
+    while (_dirtyNodes.isNotEmpty) {
+      List<SemanticsNode> localDirtyNodes = _dirtyNodes.toList();
+      _dirtyNodes.clear();
+      localDirtyNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
+      visitedNodes.addAll(localDirtyNodes);
+      for (SemanticsNode node in localDirtyNodes) {
         assert(node._dirty);
         assert(node.parent == null || !node.parent._shouldMergeAllDescendantsIntoThisNode || node._inheritedMergeAllDescendantsIntoThisNode);
         if (node._shouldMergeAllDescendantsIntoThisNode) {
@@ -557,11 +559,11 @@ class SemanticsOwner {
             }
           }
         }
-      assert(_dirtyNodes[index] == node); // make sure nothing went in front of us in the list
       }
-    _dirtyNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
+    }
+    visitedNodes.sort((SemanticsNode a, SemanticsNode b) => a.depth - b.depth);
     List<mojom.SemanticsNode> updatedNodes = <mojom.SemanticsNode>[];
-    for (SemanticsNode node in _dirtyNodes) {
+    for (SemanticsNode node in visitedNodes) {
       assert(node.parent?._dirty != true); // could be null (no parent) or false (not dirty)
       // The _serialize() method marks the node as not dirty, and
       // recurses through the tree to do a deep serialization of all

packages/flutter/test/widget/semantics_test.dart

@@ -50,4 +50,57 @@ void main() {
     client.updates.clear();
     client.dispose();
   });
+
+  testWidgets('Detach and reattach assert', (WidgetTester tester) async {
+    TestSemanticsClient client = new TestSemanticsClient(tester.binding.pipelineOwner);
+    GlobalKey key = new GlobalKey();
+
+    await tester.pumpWidget(
+      new Container(
+        child: new Semantics(
+          label: 'test1',
+          child: new Semantics(
+            key: key,
+            container: true,
+            label: 'test2a',
+            child: new Container()
+          )
+        )
+      )
+    );
+
+    expect(client.updates.length, equals(1));
+    expect(client.updates[0].strings.label, equals('test1'));
+    expect(client.updates[0].children.length, equals(1));
+    expect(client.updates[0].children[0].strings.label, equals('test2a'));
+    client.updates.clear();
+
+    await tester.pumpWidget(
+      new Container(
+        child: new Semantics(
+          label: 'test1',
+          child: new Semantics(
+            container: true,
+            label: 'middle',
+            child: new Semantics(
+              key: key,
+              container: true,
+              label: 'test2b',
+              child: new Container()
+            )
+          )
+        )
+      )
+    );
+
+    expect(client.updates.length, equals(1));
+    expect(client.updates[0].strings.label, equals('test1'));
+    expect(client.updates[0].children.length, equals(1));
+    expect(client.updates[0].children[0].strings.label, equals('middle'));
+    expect(client.updates[0].children[0].children.length, equals(1));
+    expect(client.updates[0].children[0].children[0].strings.label, equals('test2b'));
+    expect(client.updates[0].children[0].children[0].children.length, equals(0));
+
+    client.dispose();
+  });
 }