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

pkg/front_end/lib/src/fasta/source/diet_listener.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_listener;
 
 import 'package:kernel/ast.dart' show AsyncMarker, Class, InterfaceType;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
 import 'package:kernel/core_types.dart' show CoreTypes;
 
 import '../../scanner/token.dart' show BeginToken, Token;
 
 import '../builder/builder.dart';
 
 import '../deprecated_problems.dart'
     show Crash, deprecated_InputError, deprecated_inputError;
 
 import '../fasta_codes.dart'
     show Message, codeExpectedBlockToSkip, templateInternalProblemNotFound;
 
 import '../kernel/body_builder.dart' show BodyBuilder;
 
 import '../parser/native_support.dart'
     show removeNativeClause, skipNativeClause;
 
-import '../parser/parser.dart' show MemberKind, Parser, optional;
+import '../parser.dart' show MemberKind, Parser, optional;
 
 import '../problems.dart' show internalProblem;
 
 import '../type_inference/type_inference_engine.dart' show TypeInferenceEngine;
 
 import '../type_inference/type_inference_listener.dart'
     show TypeInferenceListener;
 
 import '../type_inference/type_inferrer.dart' show TypeInferrerDisabled;
 
@@ skipping 599 matching lines @@
     library.addCompileTimeError(message, charOffset, uri,
         // We assume this error has already been reported by OutlineBuilder.
         silent: true);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }