Introduce WorldImpact

pkg/compiler/lib/src/dart_backend/backend.dart

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index 93fe007f45029d9d5c4722d9f0d96de3166c4a1a..78aaeba6d7b317b34d70c266191b03b703888110 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -129,7 +129,8 @@ class DartBackend extends Backend {
     world.registerInvocation(new Selector.call("compareTo", null, 1));
   }
 
-  void codegen(CodegenWorkItem work) { }
+  WorldImpact codegen(CodegenWorkItem work) => const WorldImpact();
+
   /**
    * Tells whether we should output given element. Corelib classes like
    * Object should not be in the resulting code.