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

pkg/front_end/lib/src/fasta/source/diet_parser.dart

Index: pkg/front_end/lib/src/fasta/source/diet_parser.dart
diff --git a/pkg/front_end/lib/src/fasta/source/diet_parser.dart b/pkg/front_end/lib/src/fasta/source/diet_parser.dart
index 89226e4cec66e834be4ddc845cd12317c27568cb..821824d72f0cbb25712057b2c440f33feafc8cb2 100644
--- a/pkg/front_end/lib/src/fasta/source/diet_parser.dart
+++ b/pkg/front_end/lib/src/fasta/source/diet_parser.dart
@@ -4,15 +4,11 @@
 
 library fasta.diet_parser;
 
-import '../fasta_codes.dart' show messageExpectedOpenParens;
-
-import '../parser/class_member_parser.dart' show ClassMemberParser;
-
-import '../parser/listener.dart' show Listener;
+import '../../scanner/token.dart' show BeginToken, Token;
 
-import '../parser/parser.dart' show MemberKind, optional;
+import '../fasta_codes.dart' show messageExpectedOpenParens;
 
-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 {