Use TaskCache class from Microsoft.Extensions.TaskCache.Sources (#829)

Instead of Task.FromResult(0)

src/Microsoft.AspNetCore.Razor.Runtime/Internal/TaskCache.cs

@@ -1,24 +0,0 @@
-// 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.Threading.Tasks;
-
-namespace Microsoft.AspNetCore.Razor.Internal
-{
-    public static class TaskCache
-    {
-
-        /// <summary>
-        /// A <see cref="Task"/> that's already completed successfully.
-        /// </summary>
-        /// <remarks>
-        /// We're caching this in a static readonly field to make it more inlinable and avoid the volatile lookup done
-        /// by <c>Task.CompletedTask</c>.
-        /// </remarks>
-#if NET451
-        public static readonly Task CompletedTask = Task.FromResult(0);
-#else
-        public static readonly Task CompletedTask = Task.CompletedTask;
-#endif
-    }
-}

src/Microsoft.AspNetCore.Razor.Runtime/Runtime/TagHelpers/TagHelperExecutionContext.cs

@@ -6,8 +6,8 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Text.Encodings.Web;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Razor.Internal;
 using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.Extensions.Internal;
 
 namespace Microsoft.AspNetCore.Razor.Runtime.TagHelpers
 {

src/Microsoft.AspNetCore.Razor.Runtime/TagHelpers/TagHelper.cs

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Razor.Internal;
+using Microsoft.Extensions.Internal;
 
 namespace Microsoft.AspNetCore.Razor.TagHelpers
 {

src/Microsoft.AspNetCore.Razor.Runtime/project.json

@@ -37,6 +37,10 @@
     "Microsoft.Extensions.HashCodeCombiner.Sources": {
       "type": "build",
       "version": "1.1.0-*"
+    },
+    "Microsoft.Extensions.TaskCache.Sources": {
+      "version": "1.1.0-*",
+      "type": "build"
     }
   },
   "frameworks": {