Use a null LayoutCallbackBuilderHandle in Release

packages/flutter/lib/widgets/drawer.dart

@@ -6,8 +6,6 @@ import 'dart:sky' as sky;
 
 import 'package:sky/animation/animated_value.dart';
 import 'package:sky/animation/animation_performance.dart';
-import 'package:sky/animation/curves.dart';
-import 'package:sky/base/lerp.dart';
 import 'package:sky/theme/shadows.dart';
 import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/widgets/animated_component.dart';

packages/flutter/lib/widgets/widget.dart

@@ -631,11 +631,13 @@ int _inLayoutCallbackBuilder = 0;
 
 class LayoutCallbackBuilderHandle { bool _active = true; }
 LayoutCallbackBuilderHandle enterLayoutCallbackBuilder() {
+  LayoutCallbackBuilderHandle result;
   assert(() {
     _inLayoutCallbackBuilder += 1;
+    result = new LayoutCallbackBuilderHandle();
     return true;
   });
-  return new LayoutCallbackBuilderHandle();
+  return result;
 }
 void exitLayoutCallbackBuilder(LayoutCallbackBuilderHandle handle) {
   assert(() {