Fix crashes in scanner.

dart/sdk/lib/_internal/compiler/implementation/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.
 
 part of scanner;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
@@ skipping 33 matching lines @@
 
   Token firstToken() => tokens.next;
   Token previousToken() => tail;
 
   void appendSubstringToken(PrecedenceInfo info, int start,
                             bool asciiOnly, [int extraOffset = 0]) {
     tail.next = new StringToken.fromSubstring(info, string, start,
         scanOffset + extraOffset, tokenStart, canonicalize: true);
     tail = tail.next;
   }
+
+  bool atEndOfFile() => scanOffset >= string.length - 1;
 }