Use SubStringScanner to scan generic comments.

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

 // Copyright (c) 2011, 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 dart2js.scanner.string_scanner;
 
 import 'array_based_scanner.dart' show ArrayBasedScanner;
 
 import 'precedence.dart' show PrecedenceInfo;
 
@@ skipping 52 matching lines @@
   DartDocToken createDartDocToken(
       PrecedenceInfo info, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new DartDocToken.fromSubstring(
         info, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true);
   }
 
   bool atEndOfFile() => scanOffset >= string.length - 1;
 }
+
+/**
+ * Scanner that creates tokens for a part of a larger [String], where the part
+ * starts at the [baseOffset].
+ */
+class SubStringScanner extends StringScanner {
+  final int baseOffset;
+
+  SubStringScanner(this.baseOffset, String string,
+      {bool includeComments: false})
+      : super(string, includeComments: includeComments);
+
+  @override
+  void beginToken() {
+    tokenStart = baseOffset + stringOffset;
+  }
+}