Make RenderFlex support taking children in the constructor.

sky/sdk/lib/rendering/flex.dart

Index: sky/sdk/lib/rendering/flex.dart
diff --git a/sky/sdk/lib/rendering/flex.dart b/sky/sdk/lib/rendering/flex.dart
index 5f8f6e38991470dbc33cdc6388bee27b6e2f5806..0a6573c7391e72665b06a2407fd66900f447d49f 100644
--- a/sky/sdk/lib/rendering/flex.dart
+++ b/sky/sdk/lib/rendering/flex.dart
@@ -42,10 +42,16 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
   // lays out RenderBox children using flexible layout
 
   RenderFlex({
+    List<RenderBox> children,
     FlexDirection direction: FlexDirection.horizontal,
     FlexJustifyContent justifyContent: FlexJustifyContent.start,
     FlexAlignItems alignItems: FlexAlignItems.center
-  }) : _direction = direction, _justifyContent = justifyContent, _alignItems = alignItems;
+    }) : _direction = direction,
+         _justifyContent = justifyContent,
+         _alignItems = alignItems {
+      addAll(children);
+    }
+
 
   FlexDirection _direction;
   FlexDirection get direction => _direction;