Extract CodegenImpactTransformer from ImpactTransformer

pkg/compiler/lib/src/common/backend_api.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 dart2js.backend_api;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
 import '../common/resolution.dart' show ResolutionImpact;
 import '../constants/expressions.dart' show ConstantExpression;
@@ skipping 19 matching lines @@
   /// a constant expression.
   ConstantExpression getConstant(Node node);
 
   /// Registers [type] as instantiated.
   void registerInstantiatedType(ResolutionInterfaceType type);
 
   /// Resolves [typeName] to a type in the context of [node].
   ResolutionDartType resolveTypeFromString(Node node, String typeName);
 }
 
-/// Backend transformation methods for the world impacts.
+/// Target-specific transformation for resolution world impacts.
+///
+/// This processes target-agnostic [ResolutionImpact]s and creates [WorldImpact]
+/// in which backend/target specific impact data is added, for example: if
+/// certain feature is used that requires some helper code from the backend
+/// libraries, this will be included by the impact transformer.
 class ImpactTransformer {
   /// Transform the [ResolutionImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformResolutionImpact(
       ResolutionEnqueuer enqueuer, ResolutionImpact worldImpact) {
     return worldImpact;
   }
-
-  /// Transform the [CodegenImpact] into a [WorldImpact] adding the
-  /// backend dependencies for features used in [worldImpact].
-  WorldImpact transformCodegenImpact(CodegenImpact worldImpact) {
-    return worldImpact;
-  }
 }
 
 /// Interface for serialization of backend specific data.
 class BackendSerialization {
   const BackendSerialization();
 
   SerializerPlugin get serializer => const SerializerPlugin();
   DeserializerPlugin get deserializer => const DeserializerPlugin();
 }
 
@@ skipping 118 matching lines @@
   InterfaceType getConstantMapTypeFor(InterfaceType sourceType,
       {bool hasProtoKey: false, bool onlyStringKeys: false});
 
   /// Returns the type of the constant symbol implementation class.
   InterfaceType get symbolType;
 
   /// Returns the field of the constant symbol implementation class that holds
   /// its internal name.
   FieldEntity get symbolField;
 }