diff --git a/packages/flutter/lib/src/rendering/stack.dart b/packages/flutter/lib/src/rendering/stack.dart
index 85af118fe61..54b329b41bf 100644
--- a/packages/flutter/lib/src/rendering/stack.dart
+++ b/packages/flutter/lib/src/rendering/stack.dart
@@ -42,8 +42,8 @@ class RelativeRect {
     return new RelativeRect.fromLTRB(
       rect.left - container.left,
       rect.top - container.top,
-      container.right - rect.left + rect.width,
-      container.bottom - rect.top + rect.height
+      container.right - rect.right,
+      container.bottom - rect.bottom
     );
   }
 
diff --git a/packages/flutter/lib/src/widgets/basic.dart b/packages/flutter/lib/src/widgets/basic.dart
index 248f343a70d..ac64a03bb9b 100644
--- a/packages/flutter/lib/src/widgets/basic.dart
+++ b/packages/flutter/lib/src/widgets/basic.dart
@@ -692,8 +692,11 @@ class Positioned extends ParentDataWidget {
       needsLayout = true;
     }
 
-    if (needsLayout)
-      renderObject.markNeedsLayout();
+    if (needsLayout) {
+      AbstractNode targetParent = renderObject.parent;
+      if (targetParent is RenderObject)
+        targetParent.markNeedsLayout();
+    }
   }
 
   void debugFillDescription(List<String> description) {
@@ -788,7 +791,9 @@ class Flexible extends ParentDataWidget {
     final FlexParentData parentData = renderObject.parentData;
     if (parentData.flex != flex) {
       parentData.flex = flex;
-      renderObject.markNeedsLayout();
+      AbstractNode targetParent = renderObject.parent;
+      if (targetParent is RenderObject)
+        targetParent.markNeedsLayout();
     }
   }
 
diff --git a/packages/flutter/lib/src/widgets/transitions.dart b/packages/flutter/lib/src/widgets/transitions.dart
index bd2f7dab4a5..c696131a408 100644
--- a/packages/flutter/lib/src/widgets/transitions.dart
+++ b/packages/flutter/lib/src/widgets/transitions.dart
@@ -4,11 +4,13 @@
 
 import 'package:flutter/animation.dart';
 import 'package:vector_math/vector_math_64.dart' show Matrix4;
+import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
 import 'framework.dart';
 
 export 'package:flutter/animation.dart' show AnimationDirection;
+export 'package:flutter/rendering.dart' show RelativeRect;
 
 abstract class TransitionComponent extends StatefulComponent {
   TransitionComponent({
@@ -150,6 +152,47 @@ class SquashTransition extends TransitionWithChild {
   }
 }
 
+/// An animated variable containing a RelativeRectangle
+///
+/// This class specializes the interpolation of AnimatedValue<RelativeRect> to
+/// be appropriate for rectangles that are described in terms of offsets from
+/// other rectangles.
+class AnimatedRelativeRectValue extends AnimatedValue<RelativeRect> {
+  AnimatedRelativeRectValue(RelativeRect begin, { RelativeRect end, Curve curve, Curve reverseCurve })
+    : super(begin, end: end, curve: curve, reverseCurve: reverseCurve);
+
+  RelativeRect lerp(double t) => RelativeRect.lerp(begin, end, t);
+}
+
+/// Animated version of [Positioned].
+/// Only works if it's the child of a [Stack].
+class PositionedTransition extends TransitionWithChild {
+  PositionedTransition({
+    Key key,
+    this.rect,
+    PerformanceView performance,
+    Widget child
+  }) : super(key: key,
+             performance: performance,
+             child: child) {
+    assert(rect != null);
+  }
+
+  final AnimatedRelativeRectValue rect;
+
+  Widget buildWithChild(BuildContext context, Widget child) {
+    performance.updateVariable(rect);
+    return new Positioned(
+      top: rect.value.top,
+      right: rect.value.right,
+      bottom: rect.value.bottom,
+      left: rect.value.left,
+      child: child
+    );
+  }
+}
+
+
 typedef Widget BuilderFunction(BuildContext context);
 
 class BuilderTransition extends TransitionComponent {
diff --git a/packages/unit/test/widget/positioned_test.dart b/packages/unit/test/widget/positioned_test.dart
new file mode 100644
index 00000000000..7adecc8e58f
--- /dev/null
+++ b/packages/unit/test/widget/positioned_test.dart
@@ -0,0 +1,74 @@
+import 'package:flutter/animation.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+import 'widget_tester.dart';
+
+void main() {
+
+  test('Can animate position data', () {
+    testWidgets((WidgetTester tester) {
+
+      final AnimatedRelativeRectValue rect = new AnimatedRelativeRectValue(
+        new RelativeRect.fromRect(
+          new Rect.fromLTRB(10.0, 20.0, 20.0, 30.0),
+          new Rect.fromLTRB(0.0, 10.0, 100.0, 110.0)
+        ),
+        end: new RelativeRect.fromRect(
+          new Rect.fromLTRB(80.0, 90.0, 90.0, 100.0),
+          new Rect.fromLTRB(0.0, 10.0, 100.0, 110.0)
+        ),
+        curve: linear
+      );
+      final Performance performance = new Performance(
+        duration: const Duration(seconds: 10)
+      );
+      final List<Size> sizes = <Size>[];
+      final List<Point> positions = <Point>[];
+      final GlobalKey key = new GlobalKey();
+
+      void recordMetrics() {
+        RenderBox box = key.currentContext.findRenderObject();
+        BoxParentData boxParentData = box.parentData;
+        sizes.add(box.size);
+        positions.add(boxParentData.position);
+      }
+
+      tester.pumpWidget(
+        new Center(
+          child: new Container(
+            height: 100.0,
+            width: 100.0,
+            child: new Stack(<Widget>[
+              new PositionedTransition(
+                rect: rect,
+                performance: performance,
+                child: new Container(
+                  key: key
+                )
+              )
+            ])
+          )
+        )
+      ); // t=0
+      recordMetrics();
+      performance.play();
+      tester.pump(); // t=0 again
+      recordMetrics();
+      tester.pump(const Duration(seconds: 1)); // t=1
+      recordMetrics();
+      tester.pump(const Duration(seconds: 1)); // t=2
+      recordMetrics();
+      tester.pump(const Duration(seconds: 3)); // t=5
+      recordMetrics();
+      tester.pump(const Duration(seconds: 5)); // t=10
+      recordMetrics();
+
+      expect(sizes, equals([const Size(10.0, 10.0), const Size(10.0, 10.0), const Size(10.0, 10.0), const Size(10.0, 10.0), const Size(10.0, 10.0), const Size(10.0, 10.0)]));
+      expect(positions, equals([const Point(10.0, 10.0), const Point(10.0, 10.0), const Point(17.0, 17.0), const Point(24.0, 24.0), const Point(45.0, 45.0), const Point(80.0, 80.0)]));
+
+    });
+  });
+
+}