Rationalize Dart mojo and sky package structure

sky/framework/layouts/block.dart

-
-import 'dart:sky' as sky;
-import '../fn.dart';
-
-class BlockLayout extends LayoutContainer {
-
-  BlockLayout({
-    Object key,
-    List<UINode> children,
-    Style style,
-    String inlineStyle
-  }) : super(
-    key: key,
-    children: children,
-    style: style,
-    inlineStyle: inlineStyle
-  );
-
-  void layout(sky.Element skyNode) {
-    double y = 0.0;
-    skyNode.width = skyNode.parentNode.width;
-    skyNode.getChildNodes().forEach((child) {
-      child.layout();
-      child.x = 0.0;
-      child.y = y;
-      y += child.height;
-    });
-    skyNode.height = y;
-  }
-}