Add `IHtmlGenerator` and its default implementation

- part of #1243 (kind-of)
- mostly copied from `HtmlHelper` but refactored to
 - consistently take a `ViewContext` parameter and return a `TagBuilder`
 - provide `GenerateActionLink()` and `GenerateRouteLink()`
 - provide a separate `GenerateHiddenForCheckBox()`, allowing
   `GenerateCheckBox()` to return a `TagBuilder`
 - `GenerateForm()`'s `method` parameter is a `string`, not `FormMethod`

nits: format document, consistent line wrapping, variable name changes, ...

src/Microsoft.AspNet.Mvc.Core/AntiForgery/AntiForgery.cs

@@ -42,10 +42,10 @@ namespace Microsoft.AspNet.Mvc
         /// This method has a side effect:
         /// A response cookie is set if there is no valid cookie associated with the request.
         /// </remarks>
-        public HtmlString GetHtml([NotNull] HttpContext context)
+        public TagBuilder GetHtml([NotNull] HttpContext context)
         {
             var builder = _worker.GetFormInputElement(context);
-            return builder.ToHtmlString(TagRenderMode.SelfClosing);
+            return builder;
         }
 
         /// <summary>

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/HtmlHelper.cs

@@ -189,7 +189,8 @@ namespace Microsoft.AspNet.Mvc.Rendering
         /// <inheritdoc />
         public HtmlString AntiForgeryToken()
         {
-            return _antiForgeryInstance.GetHtml(ViewContext.HttpContext);
+            var tagBuilder = _antiForgeryInstance.GetHtml(ViewContext.HttpContext);
+            return tagBuilder.ToHtmlString(TagRenderMode.SelfClosing);
         }
 
         /// <inheritdoc />

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/IHtmlGenerator.cs

@@ -0,0 +1,180 @@
+// 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.Collections;
+using System.Collections.Generic;
+using System.Diagnostics.Contracts;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Net;
+using System.Text;
+using System.Threading.Tasks;
+using Microsoft.AspNet.Mvc.Core;
+using Microsoft.AspNet.Mvc.ModelBinding;
+using Microsoft.AspNet.Mvc.Rendering.Expressions;
+
+namespace Microsoft.AspNet.Mvc.Rendering
+{
+    /// <summary>
+    /// Contract for a service supporting <see cref="IHtmlHelper"/> and <c>ITagHelper</c> implementations.
+    /// </summary>
+    public interface IHtmlGenerator
+    {
+        string Encode(string value);
+
+        string Encode(object value);
+
+        string FormatValue(object value, string format);
+
+        TagBuilder GenerateActionLink(
+            [NotNull] string linkText,
+            string actionName,
+            string controllerName,
+            string protocol,
+            string hostname,
+            string fragment,
+            object routeValues,
+            object htmlAttributes);
+
+        TagBuilder GenerateAntiForgery([NotNull] ViewContext viewContext);
+
+        TagBuilder GenerateCheckBox(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            bool? isChecked,
+            object htmlAttributes);
+
+        /// <summary>
+        /// Generate an additional &lt;input type="hidden".../&gt; for checkboxes. This addresses scenarios where
+        /// unchecked checkboxes are not sent in the request. Sending a hidden input makes it possible to know that the
+        /// checkbox was present on the page when the request was submitted.
+        /// </summary>
+        TagBuilder GenerateHiddenForCheckbox(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name);
+
+        /// <summary>
+        /// Renders a &lt;form&gt; start tag to the response. When the user submits the form,
+        /// the request will be processed by an action method.
+        /// </summary>
+        /// <param name="actionName">The name of the action method.</param>
+        /// <param name="controllerName">The name of the controller.</param>
+        /// <param name="routeValues">
+        /// An <see cref="object"/> that contains the parameters for a route. The parameters are retrieved through
+        /// reflection by examining the properties of the <see cref="object"/>. This <see cref="object"/> is typically
+        /// created using <see cref="object"/> initializer syntax. Alternatively, an
+        /// <see cref="IDictionary{string, object}"/> instance containing the route parameters.
+        /// </param>
+        /// <param name="method">The HTTP method for processing the form, either GET or POST.</param>
+        /// <param name="htmlAttributes">An <see cref="IDictionary{string, object}"/> instance containing HTML
+        /// attributes to set for the element.</param>
+        /// <returns>
+        /// An <see cref="MvcForm"/> instance which emits the &lt;/form&gt; end tag when disposed.
+        /// </returns>
+        /// <remarks>
+        /// In this context, "renders" means the method writes its output using <see cref="ViewContext.Writer"/>.
+        /// </remarks>
+        TagBuilder GenerateForm(
+            [NotNull] ViewContext viewContext,
+            string actionName,
+            string controllerName,
+            object routeValues,
+            string method,
+            object htmlAttributes);
+
+        TagBuilder GenerateHidden(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            object value,
+            bool useViewData,
+            object htmlAttributes);
+
+        TagBuilder GenerateLabel(
+            [NotNull] ViewContext viewContext,
+            [NotNull] ModelMetadata metadata,
+            string name,
+            string labelText,
+            object htmlAttributes);
+
+        /// <remarks>
+        /// Not used directly in HtmlHelper. Exposed publicly for use in DefaultDisplayTemplates.
+        /// </remarks>
+        TagBuilder GenerateOption(SelectListItem item, string encodedText);
+
+        TagBuilder GeneratePassword(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            object value,
+            object htmlAttributes);
+
+        TagBuilder GenerateRadioButton(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            object value,
+            bool? isChecked,
+            object htmlAttributes);
+
+        TagBuilder GenerateRouteLink(
+            [NotNull] string linkText,
+            string routeName,
+            string protocol,
+            string hostName,
+            string fragment,
+            object routeValues,
+            object htmlAttributes);
+
+        TagBuilder GenerateSelect(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string optionLabel,
+            string name,
+            IEnumerable<SelectListItem> selectList,
+            bool allowMultiple,
+            object htmlAttributes);
+
+        TagBuilder GenerateTextArea(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            int rows,
+            int columns,
+            object htmlAttributes);
+
+        TagBuilder GenerateTextBox(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name,
+            object value,
+            string format,
+            object htmlAttributes);
+
+        TagBuilder GenerateValidationMessage(
+            [NotNull] ViewContext viewContext,
+            string name,
+            string message,
+            string tag,
+            object htmlAttributes);
+
+        TagBuilder GenerateValidationSummary(
+            [NotNull] ViewContext viewContext,
+            bool excludePropertyErrors,
+            string message,
+            string headerTag,
+            object htmlAttributes);
+
+        /// <remarks>
+        /// Not used directly in HtmlHelper. Exposed publicly for use in other IHtmlHelper implementations.
+        /// </remarks>
+        IEnumerable<ModelClientValidationRule> GetClientValidationRules(
+            [NotNull] ViewContext viewContext,
+            ModelMetadata metadata,
+            string name);
+    }
+}