Fix a few hints.

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 StringToken, Token;
+import 'token.dart' show 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]. */
@@ skipping 23 matching lines @@
   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;
 }