Restructure parser error handling and recovery.

pkg/dart_parser/lib/dart_parser.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.md file.
 
 library dart_parser;
 
 import 'package:dart_scanner/src/token.dart' show
     Token;
 
 import 'src/listener.dart' show
     Listener;
 
 import 'src/parser.dart' show
     Parser;
 
 import 'src/listener.dart' show
     ParserError;
 
 export 'src/parser.dart' show
-    Parser;
+    Parser,
+    closeBraceFor,
+    optional;
 
 export 'src/listener.dart' show
     Listener,
-    ParserError,
-    closeBraceFor;
+    ParserError;
 
 export 'src/error_kind.dart' show
     ErrorKind;
 
 export 'src/top_level_parser.dart' show
     TopLevelParser;
 
 export 'src/class_member_parser.dart' show
     ClassMemberParser;
 
 List<ParserError> parse(Token tokens) {
   Listener listener = new Listener();
   Parser parser = new Parser(listener);
   parser.parseUnit(tokens);
   return listener.recoverableErrors;
 }