Remove remaining uses of `childToSlot` (#64273)

The remaining uses of this pattern were all due to wanting to have
the child's slot when `Element.forgetChild()` was called. However,
when that method is called, the child's `slot` value is still valid
in the context of the parent, so the uses can just use `child.slot`.

This is the final round of cleanup from the fallout of #63269

packages/flutter/lib/src/cupertino/text_selection.dart

@@ -748,7 +748,6 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
 
   List<Element> _children;
   final Map<_CupertinoTextSelectionToolbarItemsSlot, Element> slotToChild = <_CupertinoTextSelectionToolbarItemsSlot, Element>{};
-  final Map<Element, _CupertinoTextSelectionToolbarItemsSlot> childToSlot = <Element, _CupertinoTextSelectionToolbarItemsSlot>{};
 
   // We keep a set of forgotten children to avoid O(n^2) work walking _children
   // repeatedly to remove children.
@@ -829,12 +828,11 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
 
   @override
   void forgetChild(Element child) {
-    assert(slotToChild.values.contains(child) || _children.contains(child));
+    assert(slotToChild.containsValue(child) || _children.contains(child));
     assert(!_forgottenChildren.contains(child));
     // Handle forgetting a child in children or in a slot.
-    if (childToSlot.containsKey(child)) {
-      final _CupertinoTextSelectionToolbarItemsSlot slot = childToSlot[child];
-      childToSlot.remove(child);
+    if (slotToChild.containsKey(child.slot)) {
+      final _CupertinoTextSelectionToolbarItemsSlot slot = child.slot as _CupertinoTextSelectionToolbarItemsSlot;
       slotToChild.remove(slot);
     } else {
       _forgottenChildren.add(child);
@@ -848,11 +846,9 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
       slotToChild.remove(slot);
-      childToSlot.remove(oldChild);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 
@@ -877,12 +873,11 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
   @override
   void debugVisitOnstageChildren(ElementVisitor visitor) {
     // Visit slot children.
-    childToSlot.forEach((Element child, _) {
-      if (!_shouldPaint(child) || _forgottenChildren.contains(child)) {
-        return;
+    for (final Element child in slotToChild.values) {
+      if (_shouldPaint(child) && !_forgottenChildren.contains(child)) {
+        visitor(child);
       }
-      visitor(child);
-    });
+    }
     // Visit list children.
     _children
         .where((Element child) => !_forgottenChildren.contains(child) && _shouldPaint(child))

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

@@ -2067,7 +2067,6 @@ class _RenderChipElement extends RenderObjectElement {
   _RenderChipElement(_ChipRenderWidget chip) : super(chip);
 
   final Map<_ChipSlot, Element> slotToChild = <_ChipSlot, Element>{};
-  final Map<Element, _ChipSlot> childToSlot = <Element, _ChipSlot>{};
 
   @override
   _ChipRenderWidget get widget => super.widget as _ChipRenderWidget;
@@ -2082,11 +2081,10 @@ class _RenderChipElement extends RenderObjectElement {
 
   @override
   void forgetChild(Element child) {
-    assert(slotToChild.values.contains(child));
-    assert(childToSlot.keys.contains(child));
-    final _ChipSlot slot = childToSlot[child];
-    childToSlot.remove(child);
-    slotToChild.remove(slot);
+    assert(slotToChild.containsValue(child));
+    assert(child.slot is _ChipSlot);
+    assert(slotToChild.containsKey(child.slot));
+    slotToChild.remove(child.slot);
     super.forgetChild(child);
   }
 
@@ -2095,11 +2093,9 @@ class _RenderChipElement extends RenderObjectElement {
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
       slotToChild.remove(slot);
-      childToSlot.remove(oldChild);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 
@@ -2115,12 +2111,10 @@ class _RenderChipElement extends RenderObjectElement {
     final Element oldChild = slotToChild[slot];
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
-      childToSlot.remove(oldChild);
       slotToChild.remove(slot);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 

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

@@ -1526,11 +1526,10 @@ class _RenderDecoration extends RenderBox {
   }
 }
 
-class _RenderDecorationElement extends RenderObjectElement {
-  _RenderDecorationElement(_Decorator widget) : super(widget);
+class _DecorationElement extends RenderObjectElement {
+  _DecorationElement(_Decorator widget) : super(widget);
 
   final Map<_DecorationSlot, Element> slotToChild = <_DecorationSlot, Element>{};
-  final Map<Element, _DecorationSlot> childToSlot = <Element, _DecorationSlot>{};
 
   @override
   _Decorator get widget => super.widget as _Decorator;
@@ -1545,11 +1544,10 @@ class _RenderDecorationElement extends RenderObjectElement {
 
   @override
   void forgetChild(Element child) {
-    assert(slotToChild.values.contains(child));
-    assert(childToSlot.keys.contains(child));
-    final _DecorationSlot slot = childToSlot[child];
-    childToSlot.remove(child);
-    slotToChild.remove(slot);
+    assert(slotToChild.containsValue(child));
+    assert(child.slot is _DecorationSlot);
+    assert(slotToChild.containsKey(child.slot));
+    slotToChild.remove(child.slot);
     super.forgetChild(child);
   }
 
@@ -1558,11 +1556,9 @@ class _RenderDecorationElement extends RenderObjectElement {
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
       slotToChild.remove(slot);
-      childToSlot.remove(oldChild);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 
@@ -1586,12 +1582,10 @@ class _RenderDecorationElement extends RenderObjectElement {
     final Element oldChild = slotToChild[slot];
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
-      childToSlot.remove(oldChild);
       slotToChild.remove(slot);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 
@@ -1694,7 +1688,7 @@ class _Decorator extends RenderObjectWidget {
   final bool expands;
 
   @override
-  _RenderDecorationElement createElement() => _RenderDecorationElement(this);
+  _DecorationElement createElement() => _DecorationElement(this);
 
   @override
   _RenderDecoration createRenderObject(BuildContext context) {

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

@@ -1164,7 +1164,6 @@ class _ListTileElement extends RenderObjectElement {
   _ListTileElement(_ListTile widget) : super(widget);
 
   final Map<_ListTileSlot, Element> slotToChild = <_ListTileSlot, Element>{};
-  final Map<Element, _ListTileSlot> childToSlot = <Element, _ListTileSlot>{};
 
   @override
   _ListTile get widget => super.widget as _ListTile;
@@ -1179,11 +1178,10 @@ class _ListTileElement extends RenderObjectElement {
 
   @override
   void forgetChild(Element child) {
-    assert(slotToChild.values.contains(child));
-    assert(childToSlot.keys.contains(child));
-    final _ListTileSlot slot = childToSlot[child];
-    childToSlot.remove(child);
-    slotToChild.remove(slot);
+    assert(slotToChild.containsValue(child));
+    assert(child.slot is _ListTileSlot);
+    assert(slotToChild.containsKey(child.slot));
+    slotToChild.remove(child.slot);
     super.forgetChild(child);
   }
 
@@ -1192,11 +1190,9 @@ class _ListTileElement extends RenderObjectElement {
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
       slotToChild.remove(slot);
-      childToSlot.remove(oldChild);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 
@@ -1213,12 +1209,10 @@ class _ListTileElement extends RenderObjectElement {
     final Element oldChild = slotToChild[slot];
     final Element newChild = updateChild(oldChild, widget, slot);
     if (oldChild != null) {
-      childToSlot.remove(oldChild);
       slotToChild.remove(slot);
     }
     if (newChild != null) {
       slotToChild[slot] = newChild;
-      childToSlot[newChild] = slot;
     }
   }
 

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

@@ -3561,8 +3561,10 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   /// This updates the child model such that, e.g., [visitChildren] does not
   /// walk that child anymore.
   ///
-  /// The element will still have a valid parent when this is called. After this
-  /// is called, [deactivateChild] is called to sever the link to this object.
+  /// The element will still have a valid parent when this is called, and the
+  /// child's [Element.slot] value will be valid in the context of that parent.
+  /// After this is called, [deactivateChild] is called to sever the link to
+  /// this object.
   ///
   /// The [update] is responsible for updating or creating the new child that
   /// will replace this [child].