Move parser helper classes to own files and clean them up.

pkg/front_end/lib/src/incremental/unlinked_unit.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 'dart:typed_data';
 
 import 'package:front_end/src/base/api_signature.dart';
-import 'package:front_end/src/fasta/parser/listener.dart' show Listener;
-import 'package:front_end/src/fasta/parser/parser.dart' show Parser, optional;
+import 'package:front_end/src/fasta/parser.dart'
+    show Listener, Parser, optional;
 import 'package:front_end/src/fasta/parser/top_level_parser.dart';
 import 'package:front_end/src/fasta/scanner.dart';
 import 'package:front_end/src/fasta/scanner/token_constants.dart'
     show STRING_TOKEN;
 import 'package:front_end/src/fasta/source/directive_listener.dart';
 import 'package:front_end/src/incremental/format.dart';
 
 /// Compute the [UnlinkedUnitBuilder] for the [content].
 UnlinkedUnitBuilder computeUnlinkedUnit(List<int> salt, List<int> content) {
   // Scan the content.
@@ skipping 97 matching lines @@
       return close;
     }
     return super.parseFunctionBody(token, isExpression, allowAbstract);
   }
 
   Token parseMixinApplicationRest(Token token) {
     hasMixin = true;
     return super.parseMixinApplicationRest(token);
   }
 }