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

pkg/front_end/lib/src/fasta/source/diet_parser.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 file.
 
 library fasta.diet_parser;
 
+import '../../scanner/token.dart' show BeginToken, Token;
+
 import '../fasta_codes.dart' show messageExpectedOpenParens;
 
-import '../parser/class_member_parser.dart' show ClassMemberParser;
-
-import '../parser/listener.dart' show Listener;
-
-import '../parser/parser.dart' show MemberKind, optional;
-
-import '../../scanner/token.dart' show BeginToken, Token;
+import '../parser.dart' show ClassMemberParser, Listener, MemberKind, optional;
 
 // TODO(ahe): Move this to parser package.
 class DietParser extends ClassMemberParser {
   DietParser(Listener listener) : super(listener);
 
   Token parseFormalParameters(Token token, MemberKind kind) {
     return skipFormals(token, kind);
   }
 
   Token skipFormals(Token token, MemberKind kind) {
     listener.beginOptionalFormalParameters(token);
     if (!optional('(', token)) {
       if (optional(';', token)) {
         reportRecoverableError(token, messageExpectedOpenParens);
         return token;
       }
       return reportUnexpectedToken(token).next;
     }
     BeginToken beginGroupToken = token;
     Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken, kind);
     return endToken.next;
   }
 }