Fix bug in readAsString when charset is LATIN-1 and content-length is set.

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,
@@ skipping 58 matching lines @@
       request.headers.set(HttpHeaders.ACCEPT_CHARSET, encoding.name);
     }
     HttpClientResponse response = await request.close();
     _throwIfFailed(response, uri);
     encoding ??= Encoding.getByName(response.headers.contentType?.charset);
     if (encoding == null || encoding == LATIN1) {
       // Default to LATIN-1 if no encoding found.
       // Special case LATIN-1 since it is common and doesn't need decoding.
       int length = response.contentLength;
       if (length < 0) length = 0;
-      var buffer = new Uint8Buffer(length);
+      // Create empty buffer with capacity matching contentLength.
+      var buffer = new Uint8Buffer(length)..length = 0;
       await for (var bytes in response) {
         buffer.addAll(bytes);
       }
       var byteList = buffer.buffer.asUint8List(0, buffer.length);
       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 _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);
   }
 }