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

pkg/compiler/lib/src/scanner/token.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 scanner;
 
 const int EOF_TOKEN = 0;
 
 const int KEYWORD_TOKEN = $k;
 const int IDENTIFIER_TOKEN = $a;
@@ skipping 244 matching lines @@
   BadInputToken(this.character, int charOffset)
       : super(charOffset);
 
   String toString() => "BadInputToken($character)";
 
   String get assertionMessage {
     return 'Character U+${character.toRadixString(16)} not allowed here.';
   }
 }
 
-// TODO(sigmund): delete once we enable null-aware-operators by default.
-class UnsupportedNullAwareToken extends ErrorToken {
-  final String operator;
-
-  UnsupportedNullAwareToken(this.operator, int charOffset)
-      : super(charOffset);
-
-  String toString() => "UnsupportedNullAwareToken($operator)";
-
-  String get assertionMessage => "'$operator' isn't supported without "
-      "the --enable-null-aware-operators flag.";
-
-  int get charCount => operator.length;
-}
-
 class UnterminatedToken extends ErrorToken {
   final String start;
   final int endOffset;
 
   UnterminatedToken(this.start, int charOffset, this.endOffset)
       : super(charOffset);
 
   String toString() => "UnterminatedToken($start)";
 
   String get assertionMessage => "'$start' isn't terminated.";
@@ skipping 456 matching lines @@
 
 const PrecedenceInfo STRING_INTERPOLATION_IDENTIFIER_INFO =
   const PrecedenceInfo('\$', 0,
                        STRING_INTERPOLATION_IDENTIFIER_TOKEN);
 
 const PrecedenceInfo HEXADECIMAL_INFO =
   const PrecedenceInfo('hexadecimal', 0, HEXADECIMAL_TOKEN);
 
 const PrecedenceInfo COMMENT_INFO =
   const PrecedenceInfo('comment', 0, COMMENT_TOKEN);