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..1961c56e9a8bf9fd49a73fe7fb63701248ef94fd 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);
+  scanner.scanLazyAssignmentOperators = scanLazyAssignmentOperators;

[Paul Berry, 2017/05/20 13:57:09]

Why not pass this in to the constructor like we do with includeComments and
scanGenericMethodComments?

[danrubel, 2017/05/20 21:17:04]

Good point. Done.

   return _tokenizeAndRecover(scanner, recover, source: source);
 }