Address a number of open issues.

lib/src/io_io.dart

 // Copyright (c) 2016, 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.
 
 import "dart:async" show Future, Stream;
 import "dart:convert" show Encoding, LATIN1, UTF8;
 import "dart:io" show File,
                       HttpStatus,
                       HttpClient,
                       HttpClientResponse,
                       HttpClientRequest,
+                      HttpException,
                       HttpHeaders;
 
 import "package:typed_data/typed_buffers.dart" show Uint8Buffer;
 
 /// Read the bytes of a URI as a stream of bytes.
 Stream<List<int>> readAsStream(Uri uri) async* {
   if (uri.scheme == "file") {
     yield* new File.fromUri(uri).openRead();
     return;
   }
@@ skipping 61 matching lines @@
       return new String.fromCharCodes(byteList);
     }
     return response.transform(encoding.decoder).join();
   }
   if (uri.scheme == "data") {
     return uri.data.contentAsString(encoding: encoding);
   }
   throw new UnsupportedError("Unsupported scheme: $uri");
 }
 
+HttpClient _sharedHttpClient = new HttpClient()..maxConnectionsPerHost = 6;
+
 Future<HttpClientResponse> _httpGetBytes(Uri uri) async {
-  HttpClientRequest request = await new HttpClient().getUrl(uri);
-    request.headers.set(HttpHeaders.ACCEPT, "application/octet-stream, */*");
+  HttpClientRequest request = await _sharedHttpClient.getUrl(uri);
+  request.headers.set(HttpHeaders.ACCEPT, "application/octet-stream, */*");
   return request.close();
 }
 
 void _throwIfFailed(HttpClientResponse response, Uri uri) {
   var statusCode = response.statusCode;
   if (statusCode < HttpStatus.OK || statusCode > HttpStatus.NO_CONTENT) {
     throw new HttpException(response.reasonPhrase, uri: uri);
   }
 }