Check for cycles in our various tree structures.

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

@@ -52,6 +52,13 @@ abstract class Layer {
     newLayer._previousSibling = _previousSibling;
     if (_previousSibling != null)
       newLayer._previousSibling._nextSibling = newLayer;
+    assert(() {
+      Layer node = this;
+      while (node.parent != null)
+        node = node.parent;
+      assert(node != newLayer); // indicates we are about to create a cycle
+      return true;
+    });
     newLayer._parent = _parent;
     if (_parent._firstChild == this)
       _parent._firstChild = newLayer;
@@ -182,6 +189,13 @@ class ContainerLayer extends Layer {
     assert(child._parent == null);
     assert(child._nextSibling == null);
     assert(child._previousSibling == null);
+    assert(() {
+      Layer node = this;
+      while (node.parent != null)
+        node = node.parent;
+      assert(node != child); // indicates we are about to create a cycle
+      return true;
+    });
     child._parent = this;
     child._previousSibling = _lastChild;
     if (_lastChild != null)

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

@@ -90,6 +90,13 @@ class AbstractNode {
   void adoptChild(AbstractNode child) {
     assert(child != null);
     assert(child._parent == null);
+    assert(() {
+      AbstractNode node = this;
+      while (node.parent != null)
+        node = node.parent;
+      assert(node != child); // indicates we are about to create a cycle
+      return true;
+    });
     child._parent = this;
     if (attached)
       child.attach();

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

@@ -773,6 +773,7 @@ abstract class Element<T extends Widget> implements BuildContext {
       Element newChild = _findAndActivateElement(key, newWidget);
       if (newChild != null) {
         assert(newChild._parent == null);
+        assert(() { _debugCheckForCycles(newChild); return true; });
         newChild._parent = this;
         newChild._updateDepth();
         newChild.attachRenderObject(newSlot);
@@ -782,11 +783,23 @@ abstract class Element<T extends Widget> implements BuildContext {
       }
     }
     Element newChild = newWidget.createElement();
+    assert(() { _debugCheckForCycles(newChild); return true; });
     newChild.mount(this, newSlot);
     assert(newChild._debugLifecycleState == _ElementLifecycle.active);
     return newChild;
   }
 
+  void _debugCheckForCycles(Element newChild) {
+    assert(newChild._parent == null);
+    assert(() {
+      Element node = this;
+      while (node._parent != null)
+        node = node._parent;
+      assert(node != newChild); // indicates we are about to create a cycle
+      return true;
+    });
+  }
+
   void _deactivateChild(Element child) {
     assert(child != null);
     assert(child._parent == this);

packages/flutter_sprites/lib/src/node.dart

@@ -403,6 +403,14 @@ class Node {
     assert(child._parent == null);
     assert(!(child is PhysicsGroup) || this is PhysicsGroup || this is PhysicsWorld);
 
+    assert(() {
+      Node node = this;
+      while (node.parent != null)
+        node = node.parent;
+      assert(node != child); // indicates we are about to create a cycle
+      return true;
+    });
+
     _childrenNeedSorting = true;
     _children.add(child);
     child._parent = this;