Remove unused imports.

pkg/front_end/lib/src/fasta/scanner/token.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 fasta.scanner.token;
 
-import 'dart:collection' show
-    HashSet;
-
-import 'dart:convert' show
-    UTF8;
-
 import 'keyword.dart' show
     Keyword;
 
 import 'precedence.dart' show
     BAD_INPUT_INFO,
     EOF_INFO,
     PrecedenceInfo;
 
 import 'token_constants.dart' show
     IDENTIFIER_TOKEN;
@@ skipping 339 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");