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

pkg/front_end/lib/src/fasta/parser/class_member_parser.dart

 // Copyright (c) 2011, 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.parser.class_member_parser;
 
 import '../../scanner/token.dart' show Token;
 
 import '../fasta_codes.dart' show Message;
 
+import 'assert.dart' show Assert;
+
 import 'listener.dart' show Listener;
 
-import 'parser.dart' show Assert, Parser;
+import 'parser.dart' show Parser;
 
 /// Parser similar to [TopLevelParser] but also parses class members (excluding
 /// their bodies).
 class ClassMemberParser extends Parser {
   ClassMemberParser(Listener listener) : super(listener);
 
   @override
   Token parseExpression(Token token) => skipExpression(token);
 
   @override
@@ skipping 15 matching lines @@
 
   // This method is overridden for two reasons:
   // 1. Avoid generating events for arguments.
   // 2. Avoid calling skip expression for each argument (which doesn't work).
   Token parseArgumentsOpt(Token token) => skipArgumentsOpt(token);
 
   Token parseFunctionBody(Token token, bool isExpression, bool allowAbstract) {
     return skipFunctionBody(token, isExpression, allowAbstract);
   }
 }