Fix unused imports.

pkg/front_end/test/scanner_roundtrip_test.dart

 // Copyright (c) 2017, 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.
 
 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';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'scanner_test.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ScannerTest_RoundTrip);
@@ skipping 71 matching lines @@
     super.test_scriptTag_withoutSpace();
   }
 
   @override
   @failingTest
   void test_scriptTag_withSpace() {
     // TODO(paulberry,ahe): Fasta scanner doesn't support script tag
     super.test_scriptTag_withSpace();
   }
 }