diff --git a/packages/flutter/test/widget/fractionally_sized_box_test.dart b/packages/flutter/test/widget/fractionally_sized_box_test.dart
index 623c2d34fe4..7c2f7abd9f6 100644
--- a/packages/flutter/test/widget/fractionally_sized_box_test.dart
+++ b/packages/flutter/test/widget/fractionally_sized_box_test.dart
@@ -10,7 +10,6 @@ import 'package:test/test.dart';
 void main() {
   test('FractionallySizedBox', () {
     testWidgets((WidgetTester tester) {
-      Size detectedSize;
       GlobalKey inner = new GlobalKey();
       tester.pumpWidget(new OverflowBox(
         minWidth: 0.0,
@@ -22,19 +21,14 @@ void main() {
           child: new FractionallySizedBox(
             width: 0.5,
             height: 0.25,
-            child: new SizeObserver(
-              onSizeChanged: (Size size) {
-                detectedSize = size;
-              },
-              child: new Container(
-                key: inner
-              )
+            child: new Container(
+              key: inner
             )
           )
         )
       ));
-      expect(detectedSize, equals(const Size(50.0, 25.0)));
       RenderBox box = inner.currentContext.findRenderObject();
+      expect(box.size, equals(const Size(50.0, 25.0)));
       expect(box.localToGlobal(Point.origin), equals(const Point(25.0, 37.5)));
     });
   });