Fix fortune test for dart-stream and dart-start

frameworks/Dart/dart-start/server.dart

@@ -288,7 +288,7 @@ main(List<String> args) {
           
           _fortuneCollection.find().toList().then((fortunes) {
             fortunes = fortunes.map((fortune) {
-              return new Fortune(fortune["_id"], fortune["message"]);
+              return new Fortune(fortune["_id"].toInt(), fortune["message"]);
             }).toList();
             fortunes.add(new Fortune(0, 'Additional fortune added at request time.'));
             fortunes.sort();

frameworks/Dart/dart-stream/server.dart

@@ -243,7 +243,7 @@ _fortunesMongoTest(HttpConnect connect) {
   
   return _fortuneCollection.find().toList().then((fortunes) {
     fortunes = fortunes.map((fortune) {
-      return new Fortune(fortune["_id"], fortune["message"]);
+      return new Fortune(fortune["_id"].toInt(), fortune["message"]);
     }).toList();
     fortunes.add(new Fortune(0, 'Additional fortune added at request time.'));
     fortunes.sort();