Correct gaps in PR #934

- correct XML comment typo introduced in `HtmlHelper` in 56d66c090e (bad merge)
- fix missed `null` requirement for `@Html.RadioButtonFor()` and remove buried `null` check
- add back `Environment.Newline` to `@Html.TextArea()` (was dropped though comment wasn't)

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

@@ -18,7 +18,7 @@ using Microsoft.AspNet.Mvc.Rendering.Expressions;
 namespace Microsoft.AspNet.Mvc.Rendering
 {
     /// <summary>
-    /// Default implementation of <see cref="IHtmlHelper">.
+    /// Default implementation of <see cref="IHtmlHelper"/>.
     /// </summary>
     public class HtmlHelper : IHtmlHelper, ICanHasViewContext
     {
@@ -814,11 +814,9 @@ namespace Microsoft.AspNet.Mvc.Rendering
             {
                 // RadioButtonFor() case. That API does not support passing isChecked directly.
                 Contract.Assert(!isChecked.HasValue);
-                if (value == null)
+
-                {
+                // Need a value to determine isChecked.
-                    // Need a value to determine isChecked.
+                Contract.Assert(value != null);
-                    throw new ArgumentNullException("value");
-                }
 
                 var model = metadata.Model;
                 var valueString = Convert.ToString(value, CultureInfo.CurrentCulture);
@@ -981,7 +979,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
 
             // The first newline is always trimmed when a TextArea is rendered, so we add an extra one
             // in case the value being rendered is something like "\r\nHello".
-            tagBuilder.InnerHtml = WebUtility.HtmlEncode(value);
+            tagBuilder.InnerHtml = Environment.NewLine + WebUtility.HtmlEncode(value);
 
             return tagBuilder.ToHtmlString(TagRenderMode.Normal);
         }

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

@@ -188,8 +188,10 @@ namespace Microsoft.AspNet.Mvc.Rendering
         }
 
         /// <inheritdoc />
-        public HtmlString RadioButtonFor<TProperty>([NotNull] Expression<Func<TModel, TProperty>> expression,
+        public HtmlString RadioButtonFor<TProperty>(
-            object value, object htmlAttributes)
+            [NotNull] Expression<Func<TModel, TProperty>> expression,
+            [NotNull] object value,
+            object htmlAttributes)
         {
             var metadata = GetModelMetadata(expression);
             return GenerateRadioButton(metadata, GetExpressionName(expression), value, isChecked: null,

src/Microsoft.AspNet.Mvc.Core/Rendering/HtmlHelperInputExtensions.cs

@@ -99,8 +99,10 @@ namespace Microsoft.AspNet.Mvc.Rendering
             return htmlHelper.RadioButton(name, value, isChecked, htmlAttributes: null);
         }
 
-        public static HtmlString RadioButtonFor<TModel, TProperty>([NotNull] this IHtmlHelper<TModel> htmlHelper,
+        public static HtmlString RadioButtonFor<TModel, TProperty>(
-            [NotNull] Expression<Func<TModel, TProperty>> expression, object value)
+            [NotNull] this IHtmlHelper<TModel> htmlHelper,
+            [NotNull] Expression<Func<TModel, TProperty>> expression,
+            [NotNull] object value)
         {
             return htmlHelper.RadioButtonFor(expression, value, htmlAttributes: null);
         }

src/Microsoft.AspNet.Mvc.Core/Rendering/IHtmlHelperOfT.cs

@@ -212,14 +212,15 @@ namespace Microsoft.AspNet.Mvc.Rendering
         /// An expression that identifies the object that contains the properties to render.
         /// </param>
         /// <param name="value">
-        /// If non-<c>null</c>, value to compare with current expression value to determine whether radio button is
+        /// Value to compare with current expression value to determine whether radio button is checked.
-        /// checked.
         /// </param>
         /// <param name="htmlAttributes">An object that contains the HTML attributes to set for the element.
         /// Alternatively, an <see cref="IDictionary{string, object}"/> instance containing the HTML attributes.
         /// </param>
         /// <returns>New <see cref="HtmlString"/> containing the rendered HTML.</returns>
-        HtmlString RadioButtonFor<TProperty>([NotNull] Expression<Func<TModel, TProperty>> expression, object value,
+        HtmlString RadioButtonFor<TProperty>(
+            [NotNull] Expression<Func<TModel, TProperty>> expression,
+            [NotNull] object value,
             object htmlAttributes);
 
         /// <summary>