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

pkg/compiler/lib/src/scanner/scanner_task.dart

 // Copyright (c) 2012, 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;
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name => 'Scanner';
 
@@ skipping 11 matching lines @@
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
       scanElements(compilationUnit);
     });
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
     Token tokens = new Scanner(script.file,
-        includeComments: compiler.preserveComments,
-        enableNullAwareOperators: compiler.enableNullAwareOperators).tokenize();
+        includeComments: compiler.preserveComments).tokenize();
     if (compiler.preserveComments) {
       tokens = compiler.processAndStripComments(tokens);
     }
     compiler.dietParser.dietParse(compilationUnit, tokens);
   }
 
   /**
    * Returns the tokens for the [source].
    *
    * The [StringScanner] implementation works on strings that end with a '0'
@@ skipping 19 matching lines @@
           new ElementListener(compiler, compilationUnit, idGenerator);
       PartialParser parser = new PartialParser(listener);
       try {
         parser.parseUnit(tokens);
       } on ParserError catch(_) {
         assert(invariant(compilationUnit, compiler.compilationFailed));
       }
     });
   }
 }