Remove uses of string + from API document generator.

lib/dartdoc/frog/parser.dart

 // Copyright (c) 2011, 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.
 
 // TODO(jimhug): Error recovery needs major work!
 /**
  * A simple recursive descent parser for the dart language.
  *
  * This parser is designed to be more permissive than the official
  * Dart grammar.  It is expected that many grammar errors would be
@@ skipping 423 matching lines @@
     int start = _peekToken.start;
     var factoryToken = _next();
 
     var names = [identifier()];
     while (_maybeEat(TokenKind.DOT)) {
       names.add(identifier());
     }
     if (_peekKind(TokenKind.LT)) {
       var tp = typeParameters();
       world.warning('type parameters on factories are no longer supported, '
-          + 'place them on the class instead', _makeSpan(tp[0].span.start));
+          'place them on the class instead', _makeSpan(tp[0].span.start));
     }
 
     var name = null;
     var type = null;
     if (_maybeEat(TokenKind.DOT)) {
       name = identifier();
     } else {
       if (names.length > 1) {
         name = names.removeLast();
       } else {
@@ skipping 1284 matching lines @@
   int _pos = 0;
   next() {
     var token = tokens[_pos];
     ++_pos;
     if (_pos == tokens.length) {
       parser.tokenizer = previousTokenizer;
     }
     return token;
   }
 }