Create Namer and Emitter on codegen start.

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index aef574dce26273a148fdfa7a645c28a1fbeac05f..dcf0ec9c05d301eca176288f89b7faf6e3dd0670 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -103,7 +103,7 @@ abstract class Backend extends Target {
   }
 
   /// Generates the output and returns the total size of the generated code.
-  int assembleProgram();
+  int assembleProgram(ClosedWorld closedWorld);
 
   List<CompilerTask> get tasks;
 
@@ -311,7 +311,7 @@ abstract class Backend extends Target {
 
   /// Called when the compiler starts running the codegen enqueuer. The
   /// [WorldImpact] of enabled backend features is returned.
-  WorldImpact onCodegenStart() => const WorldImpact();
+  WorldImpact onCodegenStart(ClosedWorld closedWorld) => const WorldImpact();
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;