fix #30138, synethic nodes causing crash generating source maps

tests/language_strong/nullaware_opt_test.dart

Index: tests/language_strong/nullaware_opt_test.dart
diff --git a/tests/language_strong/nullaware_opt_test.dart b/tests/language_strong/nullaware_opt_test.dart
index 1a3ef0990e38baede3c055ac64edd0db96f746fc..0436e8b2aac64106d1412e233176264a36566ad2 100644
--- a/tests/language_strong/nullaware_opt_test.dart
+++ b/tests/language_strong/nullaware_opt_test.dart
@@ -68,9 +68,22 @@ test2() {
   c?.m(bomb());
 }
 
+class Bar {
+  String s;
+}
+
+class Foo {
+  Bar _bar;
+  String str;
+
+  Foo(this._bar) : str = _bar?.s;
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
   }
+
+  Expect.equals(null, new Foo(new Bar()).str);
 }