Introduce BackendImpact to separate enqueueing from data.

pkg/compiler/lib/src/ssa/ssa.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 ssa;
 
 import 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
@@ skipping 23 matching lines @@
 import '../diagnostics/spannable.dart' show
     CURRENT_ELEMENT_SPANNABLE,
     Spannable;
 import '../elements/elements.dart';
 import '../elements/modelx.dart' show
     ConstructorBodyElementX,
     ElementX,
     VariableElementX;
 import '../io/source_information.dart';
 import '../js/js.dart' as js;
+import '../js_backend/backend_helpers.dart' show
+    BackendHelpers;
 import '../js_backend/js_backend.dart';
 import '../js_emitter/js_emitter.dart' show
     CodeEmitterTask,
     NativeEmitter;
 import '../native/native.dart' as native;
 import '../resolution/operators.dart';
 import '../resolution/semantic_visitor.dart';
 import '../resolution/send_resolver.dart' show
     SendResolverMixin;
 import '../resolution/tree_elements.dart' show
@@ skipping 21 matching lines @@
 part 'interceptor_simplifier.dart';
 part 'invoke_dynamic_specializers.dart';
 part 'nodes.dart';
 part 'optimize.dart';
 part 'types.dart';
 part 'types_propagation.dart';
 part 'validate.dart';
 part 'variable_allocator.dart';
 part 'value_range_analyzer.dart';
 part 'value_set.dart';