Switch on null-aware operators by default in analyzer.

tests/language/if_null_assignment_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.
 
 // Verify semantics of the ??= operator, including order of operations, by
 // keeping track of the operations performed.
 
-// SharedOptions=--enable-null-aware-operators
-
 import "package:expect/expect.dart";
 import "if_null_assignment_helper.dart" as h;
 
 bad() {
   Expect.fail('Should not be executed');
 }
 
 var xGetValue = null;
 
 get x {
@@ skipping 177 matching lines @@
   xGetValue = new C('x'); yGetValue = 1; zGetValue = 2;          /// 25: ok
   check(2, () => x[y] ??= z, ['x', 'y', 'x[1]', 'z', 'x[1]=2']); /// 25: continued
 
   // e1?.v ??= e2 is equivalent to ((x) => x == null ? null : x.v ??= e2)(e1).
   check(null, () => x?.v ??= bad(), ['x']); /// 26: ok
   xGetValue = new C('x'); xGetValue.vGetValue = 1; /// 27: ok
   check(1, () => x?.v ??= bad(), ['x', 'x.v']);     /// 27: continued
   xGetValue = new C('x'); yGetValue = 1;                 /// 28: ok
   check(1, () => x?.v ??= y, ['x', 'x.v', 'y', 'x.v=1']); /// 28: continued
 }