Delete RenderBlockFlow.

sky/tests/layout/custom.sky

 <parent style='background-color: lightblue;'>
   <child style='background-color: pink;'>
     <grandchild style='background-color: red; width: 25px; height: 25px;'></grandchild>
   </child>
   <child2 style='background-color: salmon; height: 25px;' />
 </parent>
 
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 
 import 'dart:async';
 import 'dart:sky';
 
 void main() {
   initUnit();
 
   var first = true;
 
   var parent = document.querySelector('parent');
   var firstChild = parent.firstElementChild;
   var secondChild = parent.lastElementChild;
 
   parent.setLayoutManager(() {
     if (first) {
-      first = false;
       parent.width = 200.0;
     } else {
       parent.width = 150.0;
     }
 
     firstChild.width = 100.0;
     firstChild.layout();
     firstChild.x = 100.0;
     firstChild.y = 50.0;
     firstChild.height = 50.0;
@@ skipping 25 matching lines @@
   };
 
   test("should have the right sizes after layout", () {
     Completer completer = new Completer();
 
     window.requestAnimationFrame((_) {
       expect(parent.offsetWidth, equals(200));
       expect(secondChild.offsetWidth, equals(200));
       assertNonChangingValues();
 
+      first = false;
       parent.setNeedsLayout();
 
       window.requestAnimationFrame((_) {
         expect(parent.offsetWidth, equals(150));
         expect(secondChild.offsetWidth, equals(150));
         assertNonChangingValues();
         completer.complete();
       });
     });
 
     return completer.future;
   });
 }
 </script>