Remove _emptyList from fn2.dart

sky/sdk/lib/framework/fn2.dart

Index: sky/sdk/lib/framework/fn2.dart
diff --git a/sky/sdk/lib/framework/fn2.dart b/sky/sdk/lib/framework/fn2.dart
index 60acbf0a7c8ce720baa249c23493bb1f521c6428..a589ba39d5aa76d8a9fbf7beb75ba67be0d3a8bc 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -530,8 +530,6 @@ class CustomPaint extends OneChildRenderObjectWrapper {
 
 }
 
-final List<UINode> _emptyList = new List<UINode>();
-
 abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
 
   // In MultiChildRenderObjectWrapper subclasses, slots are RenderObject nodes
@@ -540,10 +538,8 @@ abstract class MultiChildRenderObjectWrapper extends RenderObjectWrapper {
   MultiChildRenderObjectWrapper({
     Object key,
     List<UINode> children
-  }) : this.children = children == null ? _emptyList : children,
-  super(
-    key: key
-  ) {
+  }) : super(key: key),
+       this.children = children == null ? const [] : children {
     assert(!_debugHasDuplicateIds());
   }