Add visitor methods specific to ??= to SemanticSendVisitor.

pkg/compiler/lib/src/resolution/members.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 dart2js.resolution.members;
 
 import '../common/names.dart' show
     Selectors;
 import '../compiler.dart' show
     Compiler,
@@ skipping 3441 matching lines @@
 
         // TODO(23998): Remove these when selectors are only accessed
         // through the send structure.
         registry.setGetterSelectorInComplexSendSet(node, getterSelector);
         registry.setSelector(node, setterSelector);
         registry.setOperatorSelectorInComplexSendSet(node, operatorSelector);
 
         registry.registerDynamicInvocation(
             new UniverseSelector(operatorSelector, null));
 
-        SendStructure sendStructure =
-            new CompoundStructure(semantics, operator);
+        SendStructure sendStructure;
+        if (operator.kind == AssignmentOperatorKind.IF_NULL) {
+          sendStructure = new SetIfNullStructure(semantics);
+        } else {
+          sendStructure = new CompoundStructure(semantics, operator);
+        }
         registry.registerSendStructure(node, sendStructure);
       }
     }
     return new ResolutionResult.forElement(semantics.setter);
   }
 
   ResolutionResult visitSendSet(SendSet node) {
     if (node.isIndex) {
       // `a[b] = c`
       if (node.isSuperCall) {
@@ skipping 1167 matching lines @@
     }
     return const NoneResult();
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(Compiler compiler, Node node,
                       Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), compiler, node);
 }