Use SubStringScanner to scan generic comments.

pkg/front_end/lib/src/fasta/parser/parser.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.
 
 library fasta.parser.parser;
 
 import '../fasta_codes.dart'
     show
         FastaCode,
         FastaMessage,
@@ skipping 3816 matching lines @@
             return tokens;
           }
         }
       }
     }
     return token;
   }
 
   /// 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;
     }
     return firstToken;
   }
 
   void _injectTokenList(Token beforeToken, Token firstToken) {
     // Scanner creates a cyclic EOF token.
     Token lastToken = firstToken;
     while (lastToken.next.info != EOF_INFO) {
       lastToken = lastToken.next;
     }
     // Inject these new tokens into the stream.
     Token previous = beforeToken.previous;
     lastToken.setNext(beforeToken);
     previous.setNext(firstToken);
     beforeToken = firstToken;
   }
 }
 
 typedef FastaMessage NoArgument(Uri uri, int charOffset);
 
 typedef FastaMessage TokenArgument(Uri uri, int charOffset, Token token);
 
 typedef FastaMessage StringArgument(Uri uri, int charOffset, String string);