Rationalize Dart mojo and sky package structure

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();
-  }
-
-}