Remove flag for enabling null-aware operators. They are now on by default.

pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 1184 matching lines @@
       "Cannot assign a value to method '#{name}' "
       "in superclass '#{superclassName}'.");
 
   static const MessageKind ASSIGNING_TYPE = const MessageKind(
       "Cannot assign a value to a type.");
 
   static const MessageKind IF_NULL_ASSIGNING_TYPE = const MessageKind(
       "Cannot assign a value to a type. Note that types are never null, "
       "so this ??= assignment has no effect.",
       howToFix: "Try removing the '??=' assignment.",
-      options: const ['--enable-null-aware-operators'],
       examples: const [
           "class A {} main() { print(A ??= 3);}",
       ]);
 
   static const MessageKind VOID_NOT_ALLOWED = const MessageKind(
       "Type 'void' can't be used here because it isn't a return type.",
       howToFix: "Try removing 'void' keyword or replace it with 'var', 'final',"
           " or a type.",
       examples: const [
           "void x; main() {}",
@@ skipping 799 matching lines @@
       examples: const[
           "main(",
           "main(){",
           "main(){]}",
         ]);
 
   static const MessageKind UNTERMINATED_TOKEN = const MessageKind(
       // This is a fall-back message that shouldn't happen.
       "Incomplete token.");
 
-  static const MessageKind NULL_AWARE_OPERATORS_DISABLED = const MessageKind(
-      "Null-aware operators like '#{operator}' are currently experimental. "
-      "You can enable them using the --enable-null-aware-operators flag.");
-
   static const MessageKind EXPONENT_MISSING = const MessageKind(
       "Numbers in exponential notation should always contain an exponent"
       " (an integer number with an optional sign).",
       howToFix: "Make sure there is an exponent, and remove any whitespace "
       "before it.",
       examples: const ["""
 main() {
   var i = 1e;
 }
 """]);
@@ skipping 574 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }