Switch on null-aware operators by default in analyzer.

tests/language/if_null_assignment_static_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 static type of a ??= b is the least upper bound of the
 // static types of a and b.
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 
 // Determine whether the VM is running in checked mode.
 bool get checkedMode {
   try {
     var x = 'foo';
     int y = x;
     return false;
   } catch (_) {
     return true;
@@ skipping 147 matching lines @@
   (new ClassWithInstanceGetters()?.a ??= new B()).b; /// 39: static type warning
   if (!checkedMode) {
     (new ClassWithInstanceGetters()?.b ??= new A()).a; /// 40: ok
     Expect.throws(() => (new ClassWithInstanceGetters()?.b ??= new A()).b, noMethod); /// 41: static type warning
 
     // Exactly the same static warnings that would be caused by e1.v ??= e2 are
     // also generated in the case of e1?.v ??= e2.
     new ClassWithInstanceGetters()?.b ??= new C(); /// 42: static type warning
   }
 }