[dart2js] Remove ImpactTransformer.

Removing the indirection now that there is only one ImpactTransformer.

Change-Id: I20644113d4fce046138df4adc4b1f836d81c2523
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232964
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Joshua Litt <joshualitt@google.com>

pkg/compiler/lib/src/common/backend_api.dart

@@ -1,22 +0,0 @@
-// 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/resolution.dart' show ResolutionImpact;
-import '../universe/world_impact.dart' show WorldImpact;
-
-/// 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(ResolutionImpact worldImpact) {
-    return worldImpact;
-  }
-}

pkg/compiler/lib/src/js_backend/impact_transformer.dart

@@ -8,7 +8,6 @@ import '../universe/class_hierarchy.dart' show ClassHierarchyBuilder;
 
 import '../common.dart';
 import '../common/elements.dart';
-import '../common/backend_api.dart' show ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact;
 import '../common/resolution.dart' show ResolutionImpact;
 import '../constants/values.dart';
@@ -34,7 +33,13 @@ import 'native_data.dart';
 import 'runtime_types.dart';
 import 'runtime_types_resolution.dart';
 
-class JavaScriptImpactTransformer extends ImpactTransformer {
+/// JavaScript specific transformation for resolution world impacts.
+///
+/// This processes target-agnostic [ResolutionImpact]s and creates [WorldImpact]
+/// in which JavaScript specific impact data is added, for example: if
+/// a certain feature is used that requires some helper code from the backend
+/// libraries, this will be included by the impact transformer.
+class JavaScriptImpactTransformer {
   final ElementEnvironment _elementEnvironment;
   final CommonElements _commonElements;
   final BackendImpacts _impacts;
@@ -60,7 +65,8 @@ class JavaScriptImpactTransformer extends ImpactTransformer {
 
   DartTypes get _dartTypes => _commonElements.dartTypes;
 
-  @override
+  /// Transform the [ResolutionImpact] into a [WorldImpact] adding the
+  /// backend dependencies for features used in [worldImpact].
   WorldImpact transformResolutionImpact(ResolutionImpact worldImpact) {
     TransformedWorldImpact transformed = TransformedWorldImpact(worldImpact);
 

pkg/compiler/lib/src/kernel/kernel_strategy.dart

@@ -7,7 +7,6 @@ library dart2js.kernel.frontend_strategy;
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
-import '../common/backend_api.dart';
 import '../common/elements.dart';
 import '../common/names.dart' show Uris;
 import '../common/resolution.dart';
@@ -159,7 +158,7 @@ class KernelFrontendStrategy {
     // before creating the resolution enqueuer.
     AnnotationsData annotationsData = AnnotationsDataImpl(
         compiler.options, annotationsDataBuilder.pragmaAnnotations);
-    ImpactTransformer impactTransformer = JavaScriptImpactTransformer(
+    final impactTransformer = JavaScriptImpactTransformer(
         elementEnvironment,
         commonElements,
         impacts,
@@ -303,7 +302,7 @@ class KernelFrontendStrategy {
 class KernelWorkItemBuilder implements WorkItemBuilder {
   final CompilerTask _compilerTask;
   final KernelToElementMapImpl _elementMap;
-  final ImpactTransformer _impactTransformer;
+  final JavaScriptImpactTransformer _impactTransformer;
   final NativeMemberResolver _nativeMemberResolver;
   final AnnotationsDataBuilder _annotationsDataBuilder;
   final Map<MemberEntity, ClosureScopeModel> _closureModels;
@@ -347,7 +346,7 @@ class KernelWorkItemBuilder implements WorkItemBuilder {
 class KernelWorkItem implements WorkItem {
   final CompilerTask _compilerTask;
   final KernelToElementMapImpl _elementMap;
-  final ImpactTransformer _impactTransformer;
+  final JavaScriptImpactTransformer _impactTransformer;
   final NativeMemberResolver _nativeMemberResolver;
   final AnnotationsDataBuilder _annotationsDataBuilder;
   @override