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

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

Index: pkg/front_end/lib/src/fasta/parser.dart
diff --git a/pkg/front_end/lib/src/fasta/parser.dart b/pkg/front_end/lib/src/fasta/parser.dart
index 06e88dcc4bfbf8b56d5963a69807b8649a2fd225..9a4805fb406ec06761008e909e5984916000c7a2 100644
--- a/pkg/front_end/lib/src/fasta/parser.dart
+++ b/pkg/front_end/lib/src/fasta/parser.dart
@@ -10,19 +10,28 @@ 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/listener.dart' show Listener, ParserError;
-
-export 'parser/top_level_parser.dart' show TopLevelParser;
+export 'parser/assert.dart' show Assert;
 
 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/util.dart' show optional;
+
 List<ParserError> parse(Token tokens) {
   Listener listener = new Listener();
   Parser parser = new Parser(listener);