Add visitor methods specific to ??= to SemanticSendVisitor.

pkg/compiler/lib/src/constants/constant_constructors.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.
 
 // TODO(johnniwinther): Remove this library when all constant constructors are
 // computed during resolution.
 library dart2js.constants.constant_constructors;
 
 import '../dart_types.dart';
 import '../diagnostics/invariant.dart' show
@@ skipping 19 matching lines @@
 
 class ConstantConstructorComputer extends SemanticVisitor
     with SemanticDeclarationResolvedMixin,
          DeclarationResolverMixin,
          GetBulkMixin,
          SetBulkMixin,
          ErrorBulkMixin,
          InvokeBulkMixin,
          IndexSetBulkMixin,
          CompoundBulkMixin,
+         SetIfNullBulkMixin,
          UnaryBulkMixin,
          BaseBulkMixin,
          BinaryBulkMixin,
          PrefixBulkMixin,
          PostfixBulkMixin,
          NewBulkMixin,
          InitializerBulkMixin,
          FunctionBulkMixin,
          VariableBulkMixin
     implements SemanticDeclarationVisitor, SemanticSendVisitor {
@@ skipping 325 matching lines @@
     }
     return new IdenticalConstantExpression(
         apply(arguments.nodes.head), apply(arguments.nodes.tail.head));
   }
 
   @override
   ConstantExpression visitNamedArgument(NamedArgument node) {
     return apply(node.expression);
   }
 }