Make changing your layout manager work.

sky/tests/layout/custom.sky

Index: sky/tests/layout/custom.sky
diff --git a/sky/tests/layout/custom.sky b/sky/tests/layout/custom.sky
index 586dd26fe9da23f599c3fc9136b7b5ec776a94be..e212a6e7a018ada1a43069f829e278291f24bdbf 100644
--- a/sky/tests/layout/custom.sky
+++ b/sky/tests/layout/custom.sky
@@ -84,30 +84,39 @@ void main() {
         expect(secondChild.offsetWidth, equals(150));
         assertNonChangingValues();
 
-        parent.setLayoutManager(null);
+        parent.setLayoutManager(() {
+          parent.width = 250.0;
+        });
 
         window.requestAnimationFrame((_) {
-          expect(parent.offsetWidth, equals(300));
-          expect(parent.offsetHeight, equals(50));
-          expect(parent.offsetTop, equals(0));
-          expect(parent.offsetLeft, equals(0));
-
-          expect(firstChild.offsetWidth, equals(300));
-          expect(firstChild.offsetHeight, equals(25));
-          expect(firstChild.offsetTop, equals(0));
-          expect(firstChild.offsetLeft, equals(0));
-
-          expect(secondChild.offsetWidth, equals(300));
-          expect(secondChild.offsetHeight, equals(25));
-          expect(secondChild.offsetTop, equals(25));
-          expect(secondChild.offsetLeft, equals(0));
-
-          expect(grandChild.offsetWidth, equals(25));
-          expect(grandChild.offsetHeight, equals(25));
-          expect(grandChild.offsetTop, equals(0));
-          expect(grandChild.offsetLeft, equals(0));
-
-          completer.complete();
+          expect(parent.offsetWidth, equals(250));
+          assertNonChangingValues();
+
+          parent.setLayoutManager(null);
+
+          window.requestAnimationFrame((_) {
+            expect(parent.offsetWidth, equals(300));
+            expect(parent.offsetHeight, equals(50));
+            expect(parent.offsetTop, equals(0));
+            expect(parent.offsetLeft, equals(0));
+
+            expect(firstChild.offsetWidth, equals(300));
+            expect(firstChild.offsetHeight, equals(25));
+            expect(firstChild.offsetTop, equals(0));
+            expect(firstChild.offsetLeft, equals(0));
+
+            expect(secondChild.offsetWidth, equals(300));
+            expect(secondChild.offsetHeight, equals(25));
+            expect(secondChild.offsetTop, equals(25));
+            expect(secondChild.offsetLeft, equals(0));
+
+            expect(grandChild.offsetWidth, equals(25));
+            expect(grandChild.offsetHeight, equals(25));
+            expect(grandChild.offsetTop, equals(0));
+            expect(grandChild.offsetLeft, equals(0));
+
+            completer.complete();
+          });
         });
       });
     });