Switch on null-aware operators by default in analyzer.

tests/language/if_null_behavior_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.
 
 // Evaluation of an if-null expresion e of the form e1 ?? e2 is equivalent to
 // the evaluation of the expression ((x) => x == null ? e2 : x)(e1).  The
 // static type of e is the least upper bound of the static type of e1 and the
 // static type of e2.
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 
 class A {
   final String a;
   A(this.a);
 }
 
 class B extends A {
   B(String v) : b = v, super(v);
   final String b;
@@ skipping 24 matching lines @@
   Expect.equals('B', (new B('B') ?? nullC()).a); /// 08: ok
   Expect.equals('B', (new B('B') ?? nullC()).b); /// 09: static type warning
   Expect.throws(() => (new B('B') ?? nullC()).c, noMethod); /// 10: static type warning
   Expect.equals('C', (nullB() ?? new C('C')).a); /// 11: ok
   Expect.throws(() => (nullB() ?? new C('C')).b, noMethod); /// 12: static type warning
   Expect.equals('C', (nullB() ?? new C('C')).c); /// 13: static type warning
   Expect.throws(() => (nullB() ?? nullC()).a, noMethod); /// 14: ok
   Expect.throws(() => (nullB() ?? nullC()).b, noMethod); /// 15: static type warning
   Expect.throws(() => (nullB() ?? nullC()).c, noMethod); /// 16: static type warning
 }