Add AggregateExceptionTest (#6135)

Test for aspnet/Common#215

test/Microsoft.AspNetCore.Mvc.FunctionalTests/ErrorPageTests.cs

@@ -136,5 +136,23 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Contains("Loader Exceptions:", content);
             Assert.Contains(expectedMessage, content);
         }
+
+        [Fact]
+        public async void AggregateException_FlattensInnerExceptions()
+        {
+            // Arrange
+            var aggregateException = "AggregateException: One or more errors occurred.";
+            var nullReferenceException = "NullReferenceException: Foo cannot be null";
+            var indexOutOfRangeException = "IndexOutOfRangeException: Index is out of range";
+
+            // Act
+            var response = await Client.GetAsync("http://localhost/AggregateException");
+            var content = await response.Content.ReadAsStringAsync();
+
+            // Assert
+            Assert.Contains(aggregateException, content);
+            Assert.Contains(nullReferenceException, content);
+            Assert.Contains(indexOutOfRangeException, content);
+        }
     }
 }

test/WebSites/ErrorPageMiddlewareWebSite/AggregateExceptionController.cs

@@ -0,0 +1,33 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc;
+
+namespace ErrorPageMiddlewareWebSite
+{
+    public class AggregateExceptionController : Controller
+    {
+        [HttpGet("/AggregateException")]
+        public IActionResult Index()
+        {
+            var firstException = ThrowNullReferenceException();
+            var secondException = ThrowIndexOutOfRangeException();
+            Task.WaitAll(firstException, secondException);
+            return View();
+        }
+
+        private static async Task ThrowNullReferenceException()
+        {
+            await Task.Delay(0);
+            throw new NullReferenceException("Foo cannot be null");
+        }
+
+        private static async Task ThrowIndexOutOfRangeException()
+        {
+            await Task.Delay(0);
+            throw new IndexOutOfRangeException("Index is out of range");
+        }
+    }
+}