add fasta.scanner support for lazy assignment operators

pkg/front_end/lib/src/fasta/scanner.dart

 // Copyright (c) 2016, 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.md file.
 
 library fasta.scanner;
 
 import 'dart:convert' show UNICODE_REPLACEMENT_CHARACTER_RUNE, UTF8;
 
 import '../scanner/token.dart' show Token;
 
@@ skipping 59 matching lines @@
       includeComments: includeComments,
       scanGenericMethodComments: scanGenericMethodComments);
   return _tokenizeAndRecover(scanner, recover, bytes: bytes);
 }
 
 /// Scan/tokenize the given [source].
 /// If [recover] is null, then the [defaultRecoveryStrategy] is used.
 ScannerResult scanString(String source,
     {bool includeComments: false,
     bool scanGenericMethodComments: false,
+    bool scanLazyAssignmentOperators: false,
     Recover recover}) {
   assert(source != null, 'source must not be null');
   StringScanner scanner = new StringScanner(source,
       includeComments: includeComments,
-      scanGenericMethodComments: scanGenericMethodComments);
+      scanGenericMethodComments: scanGenericMethodComments,
+      scanLazyAssignmentOperators: scanLazyAssignmentOperators);
   return _tokenizeAndRecover(scanner, recover, source: source);
 }
 
 ScannerResult _tokenizeAndRecover(Scanner scanner, Recover recover,
     {List<int> bytes, String source}) {
   Token tokens = scanner.tokenize();
   if (scanner.hasErrors) {
     if (bytes == null) bytes = UTF8.encode(source);
     recover ??= defaultRecoveryStrategy;
     tokens = recover(bytes, tokens, scanner.lineStarts);
   }
   return new ScannerResult(tokens, scanner.lineStarts, scanner.hasErrors);
 }