Fix fn3 build errors

Add a LeafRenderObjectElement, fix a test to use the new contract.

packages/flutter/lib/src/fn3/framework.dart

@@ -466,6 +466,10 @@ class RenderObjectElement<T extends RenderObjectWidget> extends Element<T> {
   }
 }
 
+class LeafRenderObjectElement<T extends RenderObjectWidget> extends RenderObjectElement<T> {
+  LeafRenderObjectElement(T widget): super(widget);
+}
+
 final Object _uniqueChild = new Object();
 
 class OneChildRenderObjectElement<T extends OneChildRenderObjectWidget> extends RenderObjectElement<T> {

packages/unit/test/fn3/stateful_component_test.dart

@@ -10,10 +10,11 @@ class TestComponentConfig extends StatefulComponent {
   final Widget left;
   final Widget right;
 
-  TestComponentState createState() => new TestComponentState();
+  TestComponentState createState() => new TestComponentState(this);
 }
 
 class TestComponentState extends ComponentState<TestComponentConfig> {
+  TestComponentState(TestComponentConfig config): super(config);
   bool _showLeft = true;
 
   void flip() {