Move codegen registration to transformImpact method.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart_backend;
 
 import 'dart:async' show Future;
 import 'dart:math' show max;
 
 import '../../compiler.dart' show
     CompilerOutputProvider;
 import '../common.dart';
 import '../common/backend_api.dart' show
-    Backend;
+    Backend,
+    ImpactTransformer;
 import '../common/codegen.dart' show
     CodegenWorkItem;
 import '../common/names.dart' show
     Selectors,
     Uris;
 import '../common/registry.dart' show
     Registry;
 import '../common/resolution.dart' show
     Resolution,
-    ResolutionCallbacks,
-    ResolutionImpact,
-    TransformedWorldImpact;
+    ResolutionImpact;
 import '../common/tasks.dart' show
     CompilerTask;
 import '../compiler.dart' show
     Compiler;
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer;
 import '../library_loader.dart' show
     LoadedLibraries;
 import '../mirror_renamer/mirror_renamer.dart';
 import '../resolution/tree_elements.dart' show
     TreeElements,
     TreeElementMapping;
 import '../tokens/keyword.dart' show
     Keyword;
 import '../tree/tree.dart';
 import '../universe/selector.dart' show
     Selector;
 import '../universe/universe.dart' show
     UniverseSelector;
 import '../universe/world_impact.dart' show
-    WorldImpact;
+    WorldImpact,
+    TransformedWorldImpact;
 import '../util/util.dart';
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;
 
 part 'backend.dart';
 part 'renamer.dart';
 part 'placeholder_collector.dart';
 part 'outputter.dart';