add fasta.scanner support for lazy assignment operators

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

Index: pkg/front_end/lib/src/fasta/scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner.dart b/pkg/front_end/lib/src/fasta/scanner.dart
index d265c8ffb770622e7cbfe4e3b921638d5a3f9331..7251d6c9697e7ad9eb45226b92e69f0ddc52a2b3 100644
--- a/pkg/front_end/lib/src/fasta/scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner.dart
@@ -77,11 +77,13 @@ ScannerResult scan(List<int> bytes,
 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);
 }