Use SubStringScanner to scan generic comments.

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

Index: pkg/front_end/lib/src/fasta/parser/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/parser.dart b/pkg/front_end/lib/src/fasta/parser/parser.dart
index ecf6fc15339d4f334ec5a4e9a6b0bb794b508bf3..772f259aca64c557621b5bca9d814b0ffdd5596c 100644
--- a/pkg/front_end/lib/src/fasta/parser/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser/parser.dart
@@ -3834,8 +3834,7 @@ class Parser {
 
   /// Scans the given [code], and returns the tokens, otherwise returns `null`.
   Token _scanGenericMethodComment(String code, int offset) {
-    // TODO(scheglov) Let StringScanner to specify the string offset.
-    var scanner = new StringScanner(' ' * offset + code);
+    var scanner = new SubStringScanner(offset, code);
     Token firstToken = scanner.tokenize();
     if (scanner.hasErrors) {
       return null;