Set and remove the callbacks on the RenderObject when we create and remove it.

...and not when we mount and dismount.

Turns out that when we dismount, it's too late -- we've already set
renderObject to null.

We also mark the mixed viewport as dirty when it is removed from its parent.
Without this, we try to reuse the child nodes in subsequent syncs, which is a disaster.

packages/flutter/lib/widgets/mixed_viewport.dart

@@ -35,7 +35,6 @@ class MixedViewportLayoutState {
     _readOnlyChildOffsets = new UnmodifiableListView<double>(_childOffsets);
   }
 
-  Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();
   bool _dirty = true;
 
   int _firstVisibleChildIndex;
@@ -90,13 +89,33 @@ class MixedViewport extends RenderObjectWrapper {
   Object token;
   MixedViewportLayoutState layoutState;
 
+  Map<_Key, Widget> _childrenByKey = new Map<_Key, Widget>();
+
   // we don't pass the direction or offset to the render object when we create it, because
   // the render object is empty so it will not matter
   RenderBlockViewport get renderObject => super.renderObject;
-  RenderBlockViewport createNode() => new RenderBlockViewport();
+
+  RenderBlockViewport createNode() {
+    RenderBlockViewport result = new RenderBlockViewport();
+    result.callback = layout;
+    result.totalExtentCallback = _noIntrinsicDimensions;
+    result.maxCrossAxisDimensionCallback = _noIntrinsicDimensions;
+    result.minCrossAxisDimensionCallback = _noIntrinsicDimensions;
+    return result;
+  }
+
+  void remove() {
+    renderObject.callback = null;
+    renderObject.totalExtentCallback = null;
+    renderObject.maxCrossAxisDimensionCallback = null;
+    renderObject.minCrossAxisDimensionCallback = null;
+    super.remove();
+    _childrenByKey.clear();
+    layoutState._dirty = true;
+  }
 
   void walkChildren(WidgetTreeWalker walker) {
-    for (Widget child in layoutState._childrenByKey.values)
+    for (Widget child in _childrenByKey.values)
       walker(child);
   }
 
@@ -131,22 +150,6 @@ class MixedViewport extends RenderObjectWrapper {
     return null;
   }
 
-  void didMount() {
-    renderObject.callback = layout;
-    renderObject.totalExtentCallback = _noIntrinsicDimensions;
-    renderObject.maxCrossAxisDimensionCallback = _noIntrinsicDimensions;
-    renderObject.minCrossAxisDimensionCallback = _noIntrinsicDimensions;
-    super.didMount();
-  }
-
-  void didUnmount() {
-    renderObject.callback = null;
-    renderObject.totalExtentCallback = null;
-    renderObject.maxCrossAxisDimensionCallback = null;
-    renderObject.minCrossAxisDimensionCallback = null;
-    super.didUnmount();
-  }
-
   int _findIndexForOffsetBeforeOrAt(double offset) {
     final List<double> offsets = layoutState._childOffsets;
     int left = 0;
@@ -199,12 +202,12 @@ class MixedViewport extends RenderObjectWrapper {
           assert(widget != null);
           assert(widget.key != null);
           _Key key = new _Key.fromWidget(widget);
-          Widget oldWidget = layoutState._childrenByKey[key];
+          Widget oldWidget = _childrenByKey[key];
           assert(oldWidget != null);
           assert(oldWidget.renderObject.parent == renderObject);
           widget = syncChild(widget, oldWidget, renderObject.childAfter(oldWidget.renderObject));
           assert(widget != null);
-          layoutState._childrenByKey[key] = widget;
+          _childrenByKey[key] = widget;
         }
       }
     }
@@ -223,7 +226,7 @@ class MixedViewport extends RenderObjectWrapper {
     assert(newWidget != null);
     assert(newWidget.key != null);
     final _Key key = new _Key.fromWidget(newWidget);
-    Widget oldWidget = layoutState._childrenByKey[key];
+    Widget oldWidget = _childrenByKey[key];
     newWidget = syncChild(newWidget, oldWidget, _omit);
     assert(newWidget != null);
     // Update the offsets based on the newWidget's dimensions.
@@ -250,7 +253,7 @@ class MixedViewport extends RenderObjectWrapper {
       return null;
     assert(widget.key != null); // items in lists must have keys
     final _Key key = new _Key.fromWidget(widget);
-    Widget oldWidget = layoutState._childrenByKey[key];
+    Widget oldWidget = _childrenByKey[key];
     widget = syncChild(widget, oldWidget, _omit);
     if (index >= offsets.length - 1) {
       assert(index == offsets.length - 1);
@@ -288,7 +291,7 @@ class MixedViewport extends RenderObjectWrapper {
     Map<int, Widget> builtChildren = new Map<int, Widget>();
 
     final List<double> offsets = layoutState._childOffsets;
-    final Map<_Key, Widget> childrenByKey = layoutState._childrenByKey;
+    final Map<_Key, Widget> childrenByKey = _childrenByKey;
     double extent;
     if (direction == ScrollDirection.vertical) {
       extent = constraints.maxHeight;
@@ -451,7 +454,7 @@ class MixedViewport extends RenderObjectWrapper {
       }
     }
 
-    layoutState._childrenByKey = newChildren;
+    _childrenByKey = newChildren;
     layoutState._firstVisibleChildIndex = startIndex;
     layoutState._visibleChildCount = newChildren.length;
   }