Add *IndexSetIfNull methods 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.dart';
 import '../common/names.dart' show
     Selectors;
 import '../common/resolution.dart' show
@@ skipping 3196 matching lines @@
         registry.setSelector(node, setterSelector);
         registry.setOperatorSelectorInComplexSendSet(node, operatorSelector);
 
         registry.registerDynamicUse(
             new DynamicUse(getterSelector, null));
         registry.registerDynamicUse(
             new DynamicUse(setterSelector, null));
         registry.registerDynamicUse(
             new DynamicUse(operatorSelector, null));
 
-        SendStructure sendStructure =
-            new CompoundIndexSetStructure(semantics, operator);
+        SendStructure sendStructure;
+        if (operator.kind == AssignmentOperatorKind.IF_NULL) {
+          sendStructure = new IndexSetIfNullStructure(semantics);
+        } else {
+          sendStructure = new CompoundIndexSetStructure(semantics, operator);
+        }
         registry.registerSendStructure(node, sendStructure);
         return const NoneResult();
       }
     }
   }
 
   /// Handle super index operations like `super[a] = b`, `super[a] += b`, and
   /// `super[a]++`.
   // TODO(johnniwinther): Share code with [handleIndexSendSet].
   ResolutionResult handleSuperIndexSendSet(SendSet node) {
@@ skipping 102 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.registerDynamicUse(
             new DynamicUse(operatorSelector, null));
 
-        SendStructure sendStructure =
-            new CompoundIndexSetStructure(semantics, operator);
+        SendStructure sendStructure;
+        if (operator.kind == AssignmentOperatorKind.IF_NULL) {
+          sendStructure = new IndexSetIfNullStructure(semantics);
+        } else {
+          sendStructure = new CompoundIndexSetStructure(semantics, operator);
+        }
         registry.registerSendStructure(node, sendStructure);
         return const NoneResult();
       }
     }
   }
 
   /// Handle super index operations like `super.a = b`, `super.a += b`, and
   /// `super.a++`.
   // TODO(johnniwinther): Share code with [handleSuperIndexSendSet].
   ResolutionResult handleSuperSendSet(SendSet node) {
@@ skipping 1517 matching lines @@
     }
     return const NoneResult();
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(DiagnosticReporter reporter, Node node,
                       Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), reporter, node);
 }