Add visitor methods specific to ??= to SemanticSendVisitor.

pkg/compiler/lib/src/resolution/access_semantics.dart

 // Copyright (c) 2015, 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.
 
 /**
  * Code for classifying the semantics of identifiers appearing in a Dart file.
  */
 library dart2js.access_semantics;
 
-import '../common/names.dart' show
-    Names;
 import '../constants/expressions.dart';
 import '../elements/elements.dart';
 import '../dart_types.dart';
 
 /// Enum representing the different kinds of destinations which a property
 /// access or method or function invocation might refer to.
 enum AccessKind {
   /// The destination of the conditional access is an instance method, property,
   /// or field of a class, and thus must be determined dynamically.
   CONDITIONAL_DYNAMIC_PROPERTY,
@@ skipping 509 matching lines @@
 
   RedirectingFactoryConstructorAccessSemantics(
       ConstructorAccessKind kind,
       Element element,
       DartType type,
       this.effectiveTargetSemantics)
       : super(kind, element, type);
 }