Replace WebClient usage with HttpClient (#6388)

* Replace WebClient usage with HttpClient

* Fix test

* Attempt to fix sync over async

Author: ericstj

test/Microsoft.ML.AutoML.Tests/DatasetUtil.cs

@@ -8,6 +8,8 @@
 using System.IO.Compression;
 using System.Linq;
 using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
 using Microsoft.ML.Data;
 using Microsoft.ML.TestFrameworkCommon;
 
@@ -185,13 +187,13 @@ public static string DownloadImageSet(string imagesDownloadFolder)
             string fileName = "flower_photos_tiny_set_for_unit_tests.zip";
             string url = $"https://aka.ms/mlnet-resources/datasets/flower_photos_tiny_set_for_unit_test.zip";
 
-            Download(url, imagesDownloadFolder, fileName);
+            Download(url, imagesDownloadFolder, fileName).Wait();
             UnZip(Path.Combine(imagesDownloadFolder, fileName), imagesDownloadFolder);
 
             return Path.GetFileNameWithoutExtension(fileName);
         }
 
-        private static void Download(string url, string destDir, string destFileName)
+        private static async Task Download(string url, string destDir, string destFileName)
         {
             if (destFileName == null)
                 destFileName = Path.GetFileName(new Uri(url).AbsolutePath); ;
@@ -203,7 +205,16 @@ private static void Download(string url, string destDir, string destFileName)
             if (File.Exists(relativeFilePath))
                 return;
 
-            new WebClient().DownloadFile(url, relativeFilePath);
+            using (var client = new HttpClient())
+            {
+                var response = await client.GetAsync(url).ConfigureAwait(false);
+                var stream = await response.EnsureSuccessStatusCode().Content.ReadAsStreamAsync().ConfigureAwait(false);
+                var fileInfo = new FileInfo(relativeFilePath);
+                using (var fileStream = fileInfo.OpenWrite())
+                {
+                    await stream.CopyToAsync(fileStream).ConfigureAwait(false);
+                }
+            }
             return;
         }