Move native and js interop properties from the element model to the JS backend

pkg/compiler/lib/src/js_backend/js_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 js_backend;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 32 matching lines @@
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../elements/visitor.dart' show
     BaseElementVisitor;
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer,
     WorldImpact;
-import '../helpers/helpers.dart';
 import '../io/code_output.dart';
 import '../io/source_information.dart' show
     SourceInformationStrategy,
     useNewSourceInfo;
 import '../io/position_information.dart' show
     PositionSourceInformationStrategy;
 import '../io/start_end_information.dart' show
     StartEndSourceInformationStrategy;
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
@@ skipping 42 matching lines @@
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
 part 'frequency_namer.dart';
 part 'field_naming_mixin.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';