Create Namer and Emitter on codegen start.

pkg/compiler/lib/src/js_backend/frequency_namer.dart

Index: pkg/compiler/lib/src/js_backend/frequency_namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/frequency_namer.dart b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
index 9636e67e706a4b4ee19837045e4bfdcece7fbadd..0d4157bbc0c96c10d1e3f3bc6b2ac107c71bd5ad 100644
--- a/pkg/compiler/lib/src/js_backend/frequency_namer.dart
+++ b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
@@ -25,7 +25,8 @@ class FrequencyBasedNamer extends Namer
   jsAst.Name get staticsPropertyName =>
       _staticsPropertyName ??= getFreshName(instanceScope, 'static');
 
-  FrequencyBasedNamer(Compiler compiler) : super(compiler) {
+  FrequencyBasedNamer(JavaScriptBackend backend, ClosedWorld closedWorld)
+      : super(backend, closedWorld) {
     fieldRegistry = new _FieldNamingRegistry(this);
   }