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

tests/compiler/dart2js_extra/if_null2_test.dart

Index: tests/compiler/dart2js_extra/if_null2_test.dart
diff --git a/tests/language/lazy_static7_test.dart b/tests/compiler/dart2js_extra/if_null2_test.dart
similarity index 55%
copy from tests/language/lazy_static7_test.dart
copy to tests/compiler/dart2js_extra/if_null2_test.dart
index ff74912259a11aa65a3e9499f4126300a07b489d..a9923a8e773ced622b04938b114e0f6f02b3cb33 100644
--- a/tests/language/lazy_static7_test.dart
+++ b/tests/compiler/dart2js_extra/if_null2_test.dart
@@ -2,15 +2,14 @@
 // 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.
 
-import "package:expect/expect.dart";
+/// Regression for #24134: inference was not tracking ??= correctly.
+library tests.compiler.dart2js_extra.if_null2_test;
 
-var sideEffect = 0;
-var x = (() { sideEffect++; return 499; })();
+import "package:expect/expect.dart";
 
 main() {
-  if (new DateTime.now().day >= -1) {
-    x = 42;
-  }
-  Expect.equals(42, x);
-  Expect.equals(0, sideEffect);
+  var map;
+  map ??= {};
+  Expect.equals(0, map.length);
+  Expect.isTrue(map.length == 0);
 }