Make it possible to Paint elements into a display list.

sky/engine/core/dom/Element.h

Index: sky/engine/core/dom/Element.h
diff --git a/sky/engine/core/dom/Element.h b/sky/engine/core/dom/Element.h
index b497c7533eca4ead40df10cc5d589984162bc478..d0b5e1a1cb22c19eeb7b7e8dfdd6c93c78170c38 100644
--- a/sky/engine/core/dom/Element.h
+++ b/sky/engine/core/dom/Element.h
@@ -53,6 +53,7 @@ class IntSize;
 class LayoutCallback;
 class MutableStylePropertySet;
 class PaintingCallback;
+class PictureRecorder;
 class PropertySetCSSStyleDeclaration;
 class PseudoElement;
 class ShadowRoot;
@@ -298,6 +299,9 @@ public:
     const AtomicString& dir();
     void setDir(const AtomicString&);
 
+    // Dart exposed:
+    void paint(PictureRecorder*);
+
 protected:
     Element(const QualifiedName& tagName, Document*, ConstructionType);