Expose _descendantRenderObject as 'renderer'

This allows people to use localToGlobal and company.

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

@@ -425,10 +425,10 @@ abstract class Element<T extends Widget> implements BuildContext {
   T get widget => _widget;
   T _widget;
 
-  RenderObject get _descendantRenderObject {
+  RenderObject get renderObject {
     RenderObject result;
     void visit(Element element) {
-      assert(result == null);
+      assert(result == null); // this verifies that there's only one child
       if (element is RenderObjectElement)
         result = element.renderObject;
       else
@@ -800,10 +800,11 @@ class InheritedElement extends StatelessComponentElement<InheritedWidget> {
 /// Base class for instantiations of RenderObjectWidget subclasses
 abstract class RenderObjectElement<T extends RenderObjectWidget> extends Element<T> {
   RenderObjectElement(T widget)
-    : renderObject = widget.createRenderObject(), super(widget);
+    : _renderObject = widget.createRenderObject(), super(widget);
 
   /// The underlying [RenderObject] for this element
-  final RenderObject renderObject;
+  RenderObject get renderObject => _renderObject;
+  final RenderObject _renderObject;
   RenderObjectElement _ancestorRenderObjectElement;
 
   RenderObjectElement _findAncestorRenderObjectElement() {
@@ -938,7 +939,7 @@ class MultiChildRenderObjectElement<T extends MultiChildRenderObjectWidget> exte
 
   void insertChildRenderObject(RenderObject child, Element slot) {
     final renderObject = this.renderObject; // TODO(ianh): Remove this once the analyzer is cleverer
-    RenderObject nextSibling = slot?._descendantRenderObject;
+    RenderObject nextSibling = slot?.renderObject;
     assert(renderObject is ContainerRenderObjectMixin);
     renderObject.add(child, before: nextSibling);
     assert(renderObject == this.renderObject); // TODO(ianh): Remove this once the analyzer is cleverer
@@ -946,7 +947,7 @@ class MultiChildRenderObjectElement<T extends MultiChildRenderObjectWidget> exte
 
   void moveChildRenderObject(RenderObject child, dynamic slot) {
     final renderObject = this.renderObject; // TODO(ianh): Remove this once the analyzer is cleverer
-    RenderObject nextSibling = slot?._descendantRenderObject;
+    RenderObject nextSibling = slot?.renderObject;
     assert(renderObject is ContainerRenderObjectMixin);
     renderObject.move(child, before: nextSibling);
     assert(renderObject == this.renderObject); // TODO(ianh): Remove this once the analyzer is cleverer

packages/unit/test/fn3/coordinates_test.dart

+import 'package:sky/src/fn3.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+  test('Comparing coordinates', () {
+    WidgetTester tester = new WidgetTester();
+
+    Key keyA = new GlobalKey();
+    Key keyB = new GlobalKey();
+    Key keyC = new GlobalKey();
+
+    tester.pumpFrame(
+      new Stack([
+        new Positioned(
+          top: 100.0,
+          left: 100.0,
+          child: new SizedBox(
+            key: keyA,
+            width: 10.0,
+            height: 10.0
+          )
+        ),
+        new Positioned(
+          left: 100.0,
+          top: 200.0,
+          child: new SizedBox(
+            key: keyB,
+            width: 20.0,
+            height: 10.0
+          )
+        ),
+      ])
+    );
+
+    expect(tester.findElementByKey(keyA).renderObject.localToGlobal(const Point(0.0, 0.0)),
+           equals(const Point(100.0, 100.0)));
+
+    expect(tester.findElementByKey(keyB).renderObject.localToGlobal(const Point(0.0, 0.0)),
+           equals(const Point(100.0, 200.0)));
+
+    expect(tester.findElementByKey(keyB).renderObject.globalToLocal(const Point(110.0, 205.0)),
+           equals(const Point(10.0, 5.0)));
+  });
+}

packages/unit/test/fn3/widget_tester.dart

@@ -28,20 +28,22 @@ class WidgetTester {
     WidgetFlutterBinding.instance.renderViewElement.visitChildren(walk);
   }
 
-  Element findElement(bool predicate(Element widget)) {
+  Element findElement(bool predicate(Element element)) {
     try {
-      walkElements((Element widget) {
-        if (predicate(widget))
-          throw widget;
+      walkElements((Element element) {
+        if (predicate(element))
+          throw element;
       });
-    } catch (e) {
-      if (e is Element)
-        return e;
-      rethrow;
+    } on Element catch (e) {
+      return e;
     }
     return null;
   }
 
+  Element findElementByKey(Key key) {
+    return findElement((Element element) => element.widget.key == key);
+  }
+
   void pumpFrame(Widget widget) {
     runApp(widget);
     WidgetFlutterBinding.instance.beginFrame(0.0); // TODO(ianh): https://github.com/flutter/engine/issues/1084