Teach sky_shell.exe to run from a snapshot

sky/sdk/example/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/mojo/net/fetch.dart';
 import 'package:sky/mojo/asset_bundle.dart';
 
@@ skipping 41 matching lines @@
 typedef void StockDataCallback(StockData data);
 const _kChunkCount = 30;
 
 String _urlToFetch(int chunk) {
   if (rootBundle == null)
     return '../data/stock_data_${chunk}.json';
   return 'https://domokit.github.io/example/stocks/data/stock_data_${chunk}.json';
 }
 
 class StockDataFetcher {
-  int _currentChunk = 0;
+  int _nextChunk = 0;
   final StockDataCallback callback;
 
   StockDataFetcher(this.callback) {
     _fetchNextChunk();
   }
 
   void _fetchNextChunk() {
-    fetchBody(_urlToFetch(_currentChunk++)).then((Response response) {
+    fetchBody(_urlToFetch(_nextChunk++)).then((Response response) {
       String json = response.bodyAsString();
+      if (json == null) {
+        print("Failed to load stock data chunk ${_nextChunk - 1}");
+        return;
+      }
       JsonDecoder decoder = new JsonDecoder();
 
       callback(new StockData(decoder.convert(json)));
 
-      if (_currentChunk < _kChunkCount)
+      if (_nextChunk < _kChunkCount)
         _fetchNextChunk();
     });
   }
 }