Move all main methods to tool/.

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 '../../scanner.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));
-  }
-}