Switch on null-aware operators by default in analyzer.

tests/language/conditional_property_assignment_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.
 
 // Verify semantics of the ?. operator when it appears on the LHS of an
 // assignment.
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 import "conditional_access_helper.dart" as h;
 
 bad() {
   Expect.fail('Should not be executed');
 }
 
 noMethod(e) => e is NoSuchMethodError;
 
 class B {}
@@ skipping 64 matching lines @@
   Expect.throws(() => h.C?.staticField = null, noMethod); /// 17: static type warning
   Expect.throws(() => h.C?.staticField += null, noMethod); /// 18: static type warning
   Expect.throws(() => h.C?.staticField ??= null, noMethod); /// 19: static type warning
 
   // Nor can it be used to assign to toplevel properties in libraries imported
   // via prefix.
   h?.topLevelVar = null; /// 20: compile-time error
   h?.topLevelVar += null; /// 21: compile-time error
   h?.topLevelVar ??= null; /// 22: compile-time error
 }