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

pkg/compiler/lib/src/scanner/string_scanner.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;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner {
   /** The file content. */
   String string;
 
   /** The current offset in [string]. */
   int scanOffset = -1;
 
-  StringScanner(SourceFile file, {bool includeComments: false,
-                                  bool enableNullAwareOperators: false})
+  StringScanner(SourceFile file, {bool includeComments: false})
       : string = file.slowText(),
-        super(file, includeComments, enableNullAwareOperators) {
+        super(file, includeComments) {
     ensureZeroTermination();
   }
 
-  StringScanner.fromString(this.string, {bool includeComments: false,
-                                         bool enableNullAwareOperators: false})
-      : super(null, includeComments, enableNullAwareOperators) {
+  StringScanner.fromString(this.string, {bool includeComments: false})
+      : super(null, includeComments) {
     ensureZeroTermination();
   }
 
   void ensureZeroTermination() {
     if (string.isEmpty || string.codeUnitAt(string.length - 1) != 0) {
       // TODO(lry): abort instead of copying the array, or warn?
       string = string + '\x00';
     }
   }
 
@@ skipping 11 matching lines @@
 
   void appendSubstringToken(PrecedenceInfo info, int start,
                             bool asciiOnly, [int extraOffset = 0]) {
     tail.next = new StringToken.fromSubstring(info, string, start,
         scanOffset + extraOffset, tokenStart, canonicalize: true);
     tail = tail.next;
   }
 
   bool atEndOfFile() => scanOffset >= string.length - 1;
 }