fix #30138, synethic nodes causing crash generating source maps

tests/language_strong/nullaware_opt_test.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.
 //
 // VMOptions=--optimization_counter_threshold=5
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
 
@@ skipping 50 matching lines @@
   var x = 5 ?? bomb();
 }
 
 // Check that instructions without result do not crash.
 test2() {
   var c;
   c?.v;
   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);
 }