Switch on null-aware operators by default in analyzer.

tests/language/conditional_property_increment_decrement_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 in a postincrement or
 // preincrement expression (or a postdecrement or predecrement expression).
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 
 class C {
   int v;
   C(this.v);
   static var staticField;
 }
 
 class D {
   E v;
@@ skipping 45 matching lines @@
   { D d = new D(new E()); H h = ++d?.v; Expect.identical(d.v, h); } /// 12: static type warning
 
   // --e1?.v is equivalent to e1?.v += 1.
   Expect.equals(null, --nullC()?.v); /// 13: ok
   { C c = new C(1); Expect.equals(0, --c?.v); Expect.equals(0, c.v); } /// 14: ok
 
   // The static type of --e1?.v is the same as the static type of e1.v - 1.
   { D d = new D(new E()); F f = --d?.v; Expect.identical(d.v, f); } /// 15: ok
   { D d = new D(new E()); H h = --d?.v; Expect.identical(d.v, h); } /// 16: static type warning
 }