Switch on null-aware operators by default in analyzer.

tests/language/conditional_method_invocation_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 is used to invoke a method.
 
-// 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 33 matching lines @@
   Expect.throws(() => h.C?.staticMethod(), noMethod); /// 10: static type warning
 
   // Nor can it be used to access toplevel functions in libraries imported via
   // prefix.
   h?.topLevelFunction(); /// 11: compile-time error
 
   // However, '?.' can be used to access the toString method on the class Type.
   Expect.equals(C?.toString(), (C).toString()); /// 12: ok
   Expect.equals(h.C?.toString(), (h.C).toString()); /// 13: ok
 }