Remove flag for enabling null-aware operators. They are now on by default.

tests/language/nullaware_opt_test.dart

Index: tests/language/nullaware_opt_test.dart
diff --git a/tests/language/nullaware_opt_test.dart b/tests/language/nullaware_opt_test.dart
index aafb7ac5a149e0899978e04f5cbad6921e695d15..625ccc93db0acd612302c005d80b212c230f40ef 100644
--- a/tests/language/nullaware_opt_test.dart
+++ b/tests/language/nullaware_opt_test.dart
@@ -36,7 +36,7 @@ test() {
   Expect.equals(d, d ?? bomb());
 
   var e;
-  // The assginment to e is not executed since d != null.
+  // The assignment to e is not executed since d != null.
   d ??= e ??= new C(100);
   Expect.equals(null, e);
   e ??= new C(100);