Make RenderParagraph mutable, and make it fit the new RenderNode protocols

sky/tests/raw/render_box.dart

Index: sky/tests/raw/render_box.dart
diff --git a/sky/tests/raw/render_box.dart b/sky/tests/raw/render_box.dart
index 82a79783c9ed9873a83138897cc73cfe10b1129f..fafbbe50a38f5535628f332cd506e70ec6492c70 100644
--- a/sky/tests/raw/render_box.dart
+++ b/sky/tests/raw/render_box.dart
@@ -12,7 +12,10 @@ void main() {
 
   test("should size to render view", () {
     RenderSizedBox root = new RenderSizedBox(
-        new RenderDecoratedBox(new BoxDecoration(backgroundColor: 0xFF00FF00)));
+      child: new RenderDecoratedBox(
+        decoration: new BoxDecoration(backgroundColor: 0xFF00FF00)
+      )
+    );
     RenderView renderView = new RenderView(child: root);
     renderView.layout(new ViewConstraints(width: sky.view.width, height: sky.view.height));
     expect(root.size.width, equals(sky.view.width));