Remove uses of string + from API document generator.

lib/dartdoc/frog/parser.dart

Index: lib/dartdoc/frog/parser.dart
diff --git a/lib/dartdoc/frog/parser.dart b/lib/dartdoc/frog/parser.dart
index 4cb22ab8250a72ba142f3976f8a566d59fb8fcef..2ff8d33cbcca15252926e704d6bde63aa841f180 100644
--- a/lib/dartdoc/frog/parser.dart
+++ b/lib/dartdoc/frog/parser.dart
@@ -441,7 +441,7 @@ class Parser {
     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;