keep null check on virtual field

tests/language_strong/field_override_optimization_test.dart

Index: tests/language_strong/field_override_optimization_test.dart
diff --git a/tests/language_strong/field_override_optimization_test.dart b/tests/language_strong/field_override_optimization_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..573d154c4a81ac61824d6f9bacc22025b1e95d65
--- /dev/null
+++ b/tests/language_strong/field_override_optimization_test.dart
@@ -0,0 +1,33 @@
+// Copyright (c) 2017, 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.
+
+import "package:expect/expect.dart";
+import "package:meta/meta.dart";
+
+class A {
+  @virtual final bool flag = true;
+  @virtual final int x = 42;
+}
+
+class B extends A {
+  bool flag;
+  int x;
+}
+
+void main() {
+  A a = new B();
+  var exception;
+  try {
+    if (a.flag) {
+      Expect.fail('This should be unreachable');
+    } else {
+      Expect.fail('This should also be unreachable');
+    }
+  } on AssertionError catch(e) {
+    exception = e;
+  }
+  Expect.isTrue(exception is AssertionError);
+  Expect.throws(() => a.x + 8);
+}
+