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;
-
 import 'token.dart' show CommentToken, DartDocToken, StringToken;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
 class StringScanner extends ArrayBasedScanner {
   /** The file content. */
   String string;
 
   /** The current offset in [string]. */
+  @override
   int scanOffset = -1;
 
   StringScanner(String string, {bool includeComments: false})
       : string = ensureZeroTermination(string),
         super(includeComments);
 
-  static String ensureZeroTermination(String string) {

[ahe, 2017/04/04 08:17:00]

Please revert unrelated formatting changes.

-    return (string.isEmpty || string.codeUnitAt(string.length - 1) != 0)
-        // TODO(lry): abort instead of copying the array, or warn?
-        ? string + '\x00'
-        : string;
-  }
-
-  int advance() => string.codeUnitAt(++scanOffset);
-  int peek() => string.codeUnitAt(scanOffset + 1);
-
+  @override
   int get stringOffset => scanOffset;
 
-  int currentAsUnicode(int next) => next;
-
-  void handleUnicode(int startScanOffset) {}
+  @override
+  int advance() => string.codeUnitAt(++scanOffset);
 
   @override
-  StringToken createSubstringToken(
-      PrecedenceInfo info, int start, bool asciiOnly,
-      [int extraOffset = 0]) {
-    return new StringToken.fromSubstring(
-        info, string, start, scanOffset + extraOffset, tokenStart,
-        canonicalize: true);
-  }
+  bool atEndOfFile() => scanOffset >= string.length - 1;
 
   @override
   CommentToken createCommentToken(
       PrecedenceInfo info, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new CommentToken.fromSubstring(
         info, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true);
   }
 
   @override
   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;
+  @override
+  StringToken createSubstringToken(
+      PrecedenceInfo info, int start, bool asciiOnly,
+      [int extraOffset = 0]) {
+    return new StringToken.fromSubstring(
+        info, string, start, scanOffset + extraOffset, tokenStart,
+        canonicalize: true);
+  }
+
+  @override
+  int currentAsUnicode(int next) => next;
+
+  @override
+  void handleUnicode(int startScanOffset) {}
+
+  @override
+  int peek() => string.codeUnitAt(scanOffset + 1);
+
+  static String ensureZeroTermination(String string) {
+    return (string.isEmpty || string.codeUnitAt(string.length - 1) != 0)
+        // TODO(lry): abort instead of copying the array, or warn?
+        ? string + '\x00'
+        : string;
+  }
 }
+
+/**
+ * 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;
+  }
+}