update fasta StringToken.toString to make formatter happy

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 '../../scanner/token.dart' as analyzer;
 import '../../scanner/token.dart' show Token, TokenType;
 
 import 'token_constants.dart' show IDENTIFIER_TOKEN;
@@ skipping 84 matching lines @@
             decodeUtf8(data, start, end, valueOrLazySubstring.boolValue);
       }
       return valueOrLazySubstring;
     }
   }
 
   @override
   bool get isIdentifier => identical(kind, IDENTIFIER_TOKEN);
 
   @override
-  String toString() => "StringToken($lexeme)";
+  String toString() => lexeme;
 
   static final StringCanonicalizer canonicalizer = new StringCanonicalizer();
 
   static String canonicalizedString(
       String s, int start, int end, bool canonicalize) {
     if (!canonicalize) return s;
     return canonicalizer.canonicalize(s, start, end, false);
   }
 
   static String decodeUtf8(List<int> data, int start, int end, bool asciiOnly) {
@@ skipping 196 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");