[dart2js] fix inference of ??= and []??=

tests/compiler/dart2js_extra/if_null3_test.dart

Index: tests/compiler/dart2js_extra/if_null3_test.dart
diff --git a/tests/language/vm/regress_23238_test.dart b/tests/compiler/dart2js_extra/if_null3_test.dart
similarity index 50%
copy from tests/language/vm/regress_23238_test.dart
copy to tests/compiler/dart2js_extra/if_null3_test.dart
index 88250b2a9a924295387efae3bf3b086624d69668..dac99e0d0272f4060e129759a3efbfc8fc037711 100644
--- a/tests/language/vm/regress_23238_test.dart
+++ b/tests/compiler/dart2js_extra/if_null3_test.dart
@@ -2,14 +2,13 @@
 // 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.
 
+/// Regression for #24135: inference was not tracking `[]??=` correctly.
+library tests.compiler.dart2js_extra.if_null3_test;
+
 import "package:expect/expect.dart";
 
 void main() {
-  int x = 327680;
-  int r = 65536;
-  for (var i = 0; i < 200; i++) {
-    Expect.equals(r, x ~/ 5);
-    x *= 10;
-    r *= 10;
-  }
+  var map;
+  (((map ??= {})['key1'] ??= {})['key2'] ??= {})['key3'] = 'value';
+  Expect.equals('{key1: {key2: {key3: value}}}', '$map');
 }