Create Namer and Emitter on codegen start.

pkg/compiler/lib/src/js_emitter/js_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/js_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/js_emitter.dart b/pkg/compiler/lib/src/js_emitter/js_emitter.dart
index e902ab5b5f1629dd95347a8d64911d7794b32f4c..5d715a306abb82635d14a77804807da1a61cc931 100644
--- a/pkg/compiler/lib/src/js_emitter/js_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/js_emitter.dart
@@ -56,8 +56,10 @@ import '../js_backend/js_backend.dart'
         TypeVariableHandler;
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector;
-import '../universe/world_builder.dart' show SelectorConstraints;
+import '../universe/world_builder.dart'
+    show CodegenWorldBuilder, SelectorConstraints;
 import '../util/util.dart' show Setlet;
+import '../world.dart' show ClosedWorld;
 import 'full_emitter/emitter.dart' as full_js_emitter;
 import 'lazy_emitter/emitter.dart' as lazy_js_emitter;
 import 'model.dart';