Fix unused imports.

pkg/front_end/test/scanner_roundtrip_test.dart

Index: pkg/front_end/test/scanner_roundtrip_test.dart
diff --git a/pkg/front_end/test/scanner_roundtrip_test.dart b/pkg/front_end/test/scanner_roundtrip_test.dart
index 27c3956674e28b184740c2ebe245d646c23372af..a327451592a5dd7bf25419ed71c25ea9b559b883 100644
--- a/pkg/front_end/test/scanner_roundtrip_test.dart
+++ b/pkg/front_end/test/scanner_roundtrip_test.dart
@@ -3,11 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:front_end/src/fasta/analyzer/token_utils.dart';
-import 'package:front_end/src/fasta/scanner/error_token.dart' as fasta;
-import 'package:front_end/src/fasta/scanner/keyword.dart' as fasta;
-import 'package:front_end/src/fasta/scanner/string_scanner.dart' as fasta;
-import 'package:front_end/src/fasta/scanner/token.dart' as fasta;
-import 'package:front_end/src/fasta/scanner/token_constants.dart' as fasta;
 import 'package:front_end/src/scanner/errors.dart';
 import 'package:front_end/src/scanner/token.dart';
 import 'package:test/test.dart';