Switch on null-aware operators by default in analyzer.

tests/language/this_conditional_operator_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 that the ?. operator cannot be used for forwarding "this"
 // constructors.
 
-// SharedOptions=--enable-null-aware-operators
-
 class B {
   B();
   B.namedConstructor();
   var field = 1;
   method() => 1;
 
   B.forward()
     : this?.namedConstructor() /// 01: compile-time error
   ;
 
   test() {
     this?.field = 1;
     this?.field += 1;
     this?.field;
     this?.method();
   }
 }
 
 main() {
   new B.forward().test();
 }