Extract CodegenImpactTransformer from ImpactTransformer

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 c2ff777ad40e0f1f91e331c80dd5cf2069c82f5a..b61b6c332203c26f8e22b0dd4b60a98988b2afeb 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -37,7 +37,12 @@ abstract class ForeignResolver {
   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].
@@ -45,12 +50,6 @@ class ImpactTransformer {
       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.