[Effen] fix warnings

sky/examples/stocks/lib/stock_data.dart

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:convert';
 import 'dart:math';
-import 'package:sky/framework/debug/tracing.dart';
 import 'package:sky/framework/net/fetch.dart';
 
 // Snapshot from http://www.nasdaq.com/screening/company-list.aspx
 // Fetched 2/23/2014.
 // "Symbol","Name","LastSale","MarketCap","IPOyear","Sector","industry","Summary Quote",
 // Data in stock_data.json
 
 final Random _rng = new Random();
 
 class Stock {
@@ skipping 46 matching lines @@
       String json = response.bodyAsString();
       JsonDecoder decoder = new JsonDecoder();
 
       callback(new StockData(decoder.convert(json)));
 
       if (_currentChunk < _kChunkCount)
         _fetchNextChunk();
     });
   }
 }