Pass ClosedWorld directly to codegen tasks

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 dcf0ec9c05d301eca176288f89b7faf6e3dd0670..64cadf3d86697a47398c7ce4769b0d974e7bcb70 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -313,6 +313,9 @@ abstract class Backend extends Target {
   /// [WorldImpact] of enabled backend features is returned.
   WorldImpact onCodegenStart(ClosedWorld closedWorld) => const WorldImpact();
 
+  /// Called when code generation has been completed.
+  void onCodegenEnd() {}
+
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;