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

pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart

Index: pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart
diff --git a/pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart b/pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart
index f7ecea4236ee0ebb412e5ea5533ef57690afad44..983ab80a9a84b72d8b3519b06bd2d73670478171 100644
--- a/pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart
+++ b/pkg/analyzer/lib/src/fasta/analyzer_diet_listener.dart
@@ -31,7 +31,7 @@ import 'package:front_end/src/fasta/source/stack_listener.dart'
 
 import 'package:front_end/src/fasta/builder/builder.dart';
 
-import 'package:front_end/src/fasta/parser/parser.dart' show MemberKind, Parser;
+import 'package:front_end/src/fasta/parser.dart' show MemberKind, Parser;
 
 import 'package:front_end/src/scanner/token.dart' show Token;