Switch on null-aware operators by default in analyzer.

tests/language/if_null_precedence_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 '??' binds tighter than '?:' and less tightly than '||'.
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 
 assertionError(e) => e is AssertionError;
 
 // Determine whether the VM is running in checked mode.
 bool get checkedMode {
   try {
     var x = 'foo';
     int y = x;
     return false;
@@ skipping 38 matching lines @@
     // An incorrect parse of "a || (b ?? c)" would result in no checked-mode
     // error.
     Expect.throws(() => false || null ?? true, assertionError); /// 08: ok
   } else {
     // An incorrect parse of "a || (b ?? c)" would result in c being evaluated.
     int i = 0;                                                  /// 08: continued
     Expect.equals(false, false || null ?? i++ == 0);            /// 08: continued
     Expect.equals(0, i);                                        /// 08: continued
   }
 }