[Effen] Port fn.dart from the legacy sky.Node backend to the RenderNode backend, which is currently…

sky/framework/node.dart

+library node;
+
+class Node {
+
+  // Nodes always have a 'depth' greater than their ancestors'.
+  // There's no guarantee regarding depth between siblings. The depth
+  // of a node is used to ensure that nodes are processed in depth
+  // order. The 'depth' of a child can be more than one greater than
+  // the 'depth' of the parent, because the 'depth' values are never
+  // decreased: all that matters is that it's greater than the parent.
+  // Consider a tree with a root node A, a child B, and a grandchild
+  // C. Initially, A will have 'depth' 0, B 'depth' 1, and C 'depth'
+  // 2. If C is moved to be a child of A, sibling of B, then the
+  // numbers won't change. C's 'depth' will still be 2.
+
+  int _depth = 0;
+  int get depth => _depth;
+  void redepthChild(Node child) { // internal, do not call
+    assert(child._attached == _attached);
+    if (child._depth <= _depth) {
+      child._depth = _depth + 1;
+      child.redepthChildren();
+    }
+  }
+  void redepthChildren() { // internal, do not call
+    // override this in subclasses with child nodes
+    // simply call redepthChild(child) for each child
+  }
+
+  bool _attached = false;
+  bool get attached => _attached;
+  void attach() {
+    // override this in subclasses with child nodes
+    // simply call attach() for each child then call your superclass
+    _attached = true;
+    attachChildren();
+  }
+  attachChildren() { } // workaround for lack of inter-class mixins in Dart
+  void detach() {
+    // override this in subclasses with child nodes
+    // simply call detach() for each child then call your superclass
+    _attached = false;
+    detachChildren();
+  }
+  detachChildren() { } // workaround for lack of inter-class mixins in Dart
+
+  void setAsChild(Node child) { // only for use by subclasses
+    assert(child != null);
+    if (attached)
+      child.attach();
+    redepthChild(child);
+  }
+  void dropChild(Node child) { // only for use by subclasses
+    assert(child != null);
+    assert(child.attached == attached);
+    if (attached)
+      child.detach();
+  }
+
+}