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

tests/language/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.
 //
 // SharedOptions=--enable-null-aware-operators
 // VMOptions=--optimization_counter_threshold=5
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
@@ skipping 18 matching lines @@
   Expect.equals(null, getNull()?.anything(bomb()));
   Expect.equals(1, d?.m(1));
   Expect.equals("C", C?.toString());
 
   Expect.equals(1, new C(1)?.f);
   Expect.equals(null, c?.v);
   Expect.equals(10, c ?? 10);
   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);
   Expect.equals(100, e?.f);
   e?.f ??= 200;
   Expect.equals(100, e?.f);
 
   e.f = null;
   e?.f ??= 200;
   Expect.equals(200, e?.f);
@@ skipping 13 matching lines @@
   c?.v;
   c?.m(bomb());
 }
 
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
   }
 }