Remove package imports from itself.

pkg/front_end/lib/src/fasta/scanner/testing/scanner_chain.dart

 // Copyright (c) 2016, 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.md file.
 
 library fasta.scanner.testing.scanner_chain;
 
 import 'package:testing/testing.dart';
 
-import 'package:front_end/src/fasta/scanner.dart';
+import '../../scanner.dart';
 
-import 'package:front_end/src/fasta/scanner/io.dart';
+import '../io.dart';
 
 class Read extends Step<TestDescription, List<int>, ChainContext> {
   const Read();
 
   String get name => "read";
 
   Future<Result<List<int>>> run(
       TestDescription input, ChainContext context) async {
     return pass(await readBytesFromFile(input.uri));
   }
 }
 
 class Scan extends Step<List<int>, ScannerResult, ChainContext> {
   const Scan();
 
   String get name => "scan";
 
   Future<Result<ScannerResult>> run(
       List<int> bytes, ChainContext context) async {
     return pass(scan(bytes));
   }
 }