Add *IndexSetIfNull methods to SemanticSendVisitor.

tests/compiler/dart2js/semantic_visitor_test_send_data.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.
 
 part of dart2js.semantics_visitor_test;
 
 const Map<String, List<Test>> SEND_TESTS = const {
   'Parameters': const [
     // Parameters
     const Test('m(o) => o;',
@@ skipping 2124 matching lines @@
           operator []=(a, b) {}
         }
         class C extends B {
           static m() => super[42]--;
         }
         ''',
         const Visit(VisitKind.ERROR_INVALID_INDEX_POSTFIX,
                     error: MessageKind.NO_SUPER_IN_STATIC,
                     index: '42',
                     operator: '--')),
+    const Test(
+        '''
+        m() => [][42] ??= 0;
+        ''',
+        const Visit(VisitKind.VISIT_INDEX_SET_IF_NULL,
+                    receiver: '[] ',
+                    index: '42',
+                    rhs: '0')),
+    const Test.clazz(
+        '''
+        class B {
+          operator [](_) => null;
+          operator []=(a, b) {}
+        }
+        class C extends B {
+          m() => super[42] ??= 0;
+        }
+        ''',
+        const Visit(VisitKind.VISIT_SUPER_INDEX_SET_IF_NULL,
+                    getter: 'function(B#[])',
+                    setter: 'function(B#[]=)',
+                    index: '42',
+                    rhs: '0')),
+    const Test.clazz(
+        '''
+        class B {
+          operator []=(a, b) {}
+        }
+        class C extends B {
+          m() => super[42] ??= 0;
+        }
+        ''',
+        const Visit(VisitKind.VISIT_UNRESOLVED_SUPER_GETTER_INDEX_SET_IF_NULL,
+                    setter: 'function(B#[]=)',
+                    index: '42',
+                    rhs: '0')),
+    const Test.clazz(
+        '''
+        class B {
+          operator [](_) => null;
+        }
+        class C extends B {
+          m() => super[42] ??= 0;
+        }
+        ''',
+        const Visit(VisitKind.VISIT_UNRESOLVED_SUPER_SETTER_INDEX_SET_IF_NULL,
+                    getter: 'function(B#[])',
+                    index: '42',
+                    rhs: '0')),
+    const Test.clazz(
+        '''
+        class B {
+        }
+        class C extends B {
+          m() => super[42] ??= 0;
+        }
+        ''',
+        const Visit(VisitKind.VISIT_UNRESOLVED_SUPER_INDEX_SET_IF_NULL,
+                    index: '42',
+                    rhs: '0')),
   ],
   'Equals': const [
     // Equals
     const Test(
         '''
         m() => 2 == 3;
         ''',
         const Visit(VisitKind.VISIT_EQUALS,
                     left: '2', right: '3')),
     const Test.clazz(
@@ skipping 2357 matching lines @@
             element: 'field(a)', rhs: '42')),
 
     const Test(
         '''
         m() => unresolved ??= 42;
         ''',
         const Visit(VisitKind.VISIT_UNRESOLVED_SET_IF_NULL,
             name: 'unresolved', rhs: '42')),
   ],
 };