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

pkg/front_end/lib/src/fasta/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.md file.
 
 library fasta.parser;
 
 import '../scanner/token.dart' show Token;
 
 import 'parser/listener.dart' show Listener;
 
 import 'parser/parser.dart' show Parser;
 
-import 'parser/listener.dart' show ParserError;
+import 'parser/parser_error.dart' show ParserError;
 
-export 'parser/parser.dart'
-    show FormalParameterType, MemberKind, Parser, optional;
+export 'parser/assert.dart' show Assert;
 
-export 'parser/listener.dart' show Listener, ParserError;
+export 'parser/class_member_parser.dart' show ClassMemberParser;
+
+export 'parser/formal_parameter_kind.dart' show FormalParameterKind;
+
+export 'parser/identifier_context.dart' show IdentifierContext;
+
+export 'parser/listener.dart' show Listener;
+
+export 'parser/member_kind.dart' show MemberKind;
+
+export 'parser/parser.dart' show Parser;
+
+export 'parser/parser_error.dart' show ParserError;
 
 export 'parser/top_level_parser.dart' show TopLevelParser;
 
-export 'parser/class_member_parser.dart' show ClassMemberParser;
-
-export 'parser/identifier_context.dart' show IdentifierContext;
+export 'parser/util.dart' show optional;
 
 List<ParserError> parse(Token tokens) {
   Listener listener = new Listener();
   Parser parser = new Parser(listener);
   parser.parseUnit(tokens);
   return listener.recoverableErrors;
 }