Allow parents of RenderObjects that have layout callbacks to depend on those objects' sizes.

Previously, if you used a layout callback, you could not have a parent
that did parentUsesSize, or if you did, you had to be marked
sizedByParent. With this patch, we allow the parent to depend on your
layout, even if you modify your child list during your layout using a
layout callback, by checking that the parent is still actively being
laid out in this scenario.

packages/flutter/lib/rendering/object.dart

@@ -358,14 +358,13 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   bool get debugDoingThisLayout => _debugDoingThisLayout;
   static RenderObject _debugActiveLayout = null;
   static RenderObject get debugActiveLayout => _debugActiveLayout;
-  bool _debugDoingThisLayoutWithCallback = false;
   bool _debugMutationsLocked = false;
   bool _debugCanParentUseSize;
   bool get debugCanParentUseSize => _debugCanParentUseSize;
   bool get debugCanPerformMutations {
     RenderObject node = this;
     while (true) {
-      if (node._debugDoingThisLayoutWithCallback)
+      if (node._doingThisLayoutWithCallback)
         return true;
       if (node._debugMutationsLocked)
         return false;
@@ -379,6 +378,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   bool _needsLayout = true;
   bool get needsLayout => _needsLayout;
   RenderObject _relayoutSubtreeRoot;
+  bool _doingThisLayoutWithCallback = false;
   Constraints _constraints;
   Constraints get constraints => _constraints;
   bool debugDoesMeetConstraints(); // override this in a subclass to verify that your state matches the constraints object
@@ -390,7 +390,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       assert(node._relayoutSubtreeRoot == _relayoutSubtreeRoot);
       assert(node.parent != null);
       node = node.parent as RenderObject;
-      if (!node._needsLayout)
+      if ((!node._needsLayout) && (!node._debugDoingThisLayout))
         return false;
     }
     assert(node._relayoutSubtreeRoot == node);
@@ -407,7 +407,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     if (_relayoutSubtreeRoot != this) {
       final parent = this.parent; // TODO(ianh): Remove this once the analyzer is cleverer
       assert(parent is RenderObject);
-      parent.markNeedsLayout();
+      if (!_doingThisLayoutWithCallback) {
+        parent.markNeedsLayout();
+      } else {
+        assert(parent._debugDoingThisLayout);
+      }
       assert(parent == this.parent); // TODO(ianh): Remove this once the analyzer is cleverer
     } else {
       _nodesNeedingLayout.add(this);
@@ -458,7 +462,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       assert(_relayoutSubtreeRoot == this);
       RenderObject debugPreviousActiveLayout;
       assert(!_debugMutationsLocked);
-      assert(!_debugDoingThisLayoutWithCallback);
+      assert(!_doingThisLayoutWithCallback);
       assert(_debugCanParentUseSize != null);
       assert(() {
         _debugMutationsLocked = true;
@@ -494,7 +498,7 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
     _constraints = constraints;
     _relayoutSubtreeRoot = relayoutSubtreeRoot;
     assert(!_debugMutationsLocked);
-    assert(!_debugDoingThisLayoutWithCallback);
+    assert(!_doingThisLayoutWithCallback);
     assert(() {
       _debugMutationsLocked = true;
       _debugCanParentUseSize = parentUsesSize;
@@ -546,16 +550,13 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
   void invokeLayoutCallback(LayoutCallback callback) {
     assert(_debugMutationsLocked);
     assert(_debugDoingThisLayout);
-    assert(!_debugDoingThisLayoutWithCallback);
-    assert(() {
-      _debugDoingThisLayoutWithCallback = true;
-      return true;
-    });
-    callback(constraints);
-    assert(() {
-      _debugDoingThisLayoutWithCallback = false;
-      return true;
-    });
+    assert(!_doingThisLayoutWithCallback);
+    _doingThisLayoutWithCallback = true;
+    try {
+      callback(constraints);
+    } finally {
+      _doingThisLayoutWithCallback = false;
+    }
   }
 
   // when the parent has rotated (e.g. when the screen has been turned