Added the EnvironmentTagHelper:

- #1553

Author: DamianEdwards

src/Microsoft.AspNet.Mvc.TagHelpers/EnvironmentTagHelper.cs

@@ -0,0 +1,59 @@
+// Copyright (c) Microsoft Open Technologies, Inc. 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.Linq;
+using Microsoft.AspNet.Hosting;
+using Microsoft.AspNet.Razor.Runtime.TagHelpers;
+
+namespace Microsoft.AspNet.Mvc.TagHelpers
+{
+    /// <summary>
+    /// <see cref="ITagHelper"/> implementation targeting &lt;environment&gt; elements that conditionally renders content
+    /// based on the current value of <see cref="IHostingEnvironment.EnvironmentName"/>.
+    /// </summary>
+    public class EnvironmentTagHelper : TagHelper
+    {
+        private static readonly char[] _nameSeparator = new[] { ',' };
+
+        /// <summary>
+        /// A comma separated list of environment names in which the content should be rendered.
+        /// </summary>
+        public string Names { get; set; }
+
+        // Protected to ensure subclasses are correctly activated. Internal for ease of use when testing.
+        [Activate]
+        protected internal IHostingEnvironment HostingEnvironment { get; set; }
+
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            // Always strip the outer tag name as we never want <environment> to render
+            output.TagName = null;
+
+            if (string.IsNullOrWhiteSpace(Names))
+            {
+                // No names specified, do nothing
+                return;
+            }
+
+            var environments = Names.Split(_nameSeparator, StringSplitOptions.RemoveEmptyEntries);
+
+            if (environments.Length == 0)
+            {
+                // No names specified, do nothing
+                return;
+            }
+
+            var currentEnvironmentName = HostingEnvironment.EnvironmentName.Trim();
+            
+            if (environments.Any(name => string.Equals(name.Trim(), currentEnvironmentName, StringComparison.OrdinalIgnoreCase)))
+            {
+                // Matching environment name found, do nothing
+                return;
+            }
+            
+            // No matching environment name found, supress all output
+            output.SupressOutput();
+        }
+    }
+}