update fasta StringToken.toString to make formatter happy

pkg/front_end/lib/src/fasta/scanner/token.dart

Index: pkg/front_end/lib/src/fasta/scanner/token.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/token.dart b/pkg/front_end/lib/src/fasta/scanner/token.dart
index 3ab4c0cd7a1f4446782e5b981a4b816879b9d7ad..afbe541f58289c7073d86a7812bb6d00f9ea7df1 100644
--- a/pkg/front_end/lib/src/fasta/scanner/token.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/token.dart
@@ -102,7 +102,7 @@ class StringToken extends analyzer.TokenWithComment
   bool get isIdentifier => identical(kind, IDENTIFIER_TOKEN);
 
   @override
-  String toString() => "StringToken($lexeme)";
+  String toString() => lexeme;
 
   static final StringCanonicalizer canonicalizer = new StringCanonicalizer();