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

pkg/compiler/lib/src/scanner/array_based_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;
 
 abstract class ArrayBasedScanner extends AbstractScanner {
-  ArrayBasedScanner(SourceFile file, bool includeComments,
-                    bool enableNullAwareOperators)
-      : super(file, includeComments, enableNullAwareOperators);
+  ArrayBasedScanner(SourceFile file, bool includeComments)
+      : super(file, includeComments);
 
   /**
    * The stack of open groups, e.g [: { ... ( .. :]
    * Each BeginGroupToken has a pointer to the token where the group
    * ends. This field is set when scanning the end group token.
    */
   Link<BeginGroupToken> groupingStack = const Link<BeginGroupToken>();
 
   /**
    * Appends a fixed token whose kind and content is determined by [info].
@@ skipping 191 matching lines @@
    * something which cannot possibly be part of a type parameter/argument
    * list, like the '=' in the above example.
    */
   void discardOpenLt() {
     while (!groupingStack.isEmpty
         && identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 }