Correct some `ViewDataDictionary` constructor calls

- ensure correct `ViewData.ModelState` value in a `RazorPage` instance
- let view components inherit `ViewData.Model` from surrounding context
- do not create an isolated `ViewDataDictionary<TModel>` in one HTML helper
 - allows previous `ViewDataDictionary<TModel>` constructor to be `internal`
  - also add comments for that constructor
- change two `ViewDataDictionary` copy constructor calls to avoid uselessly
  copying `Model` and related information from source

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

@@ -98,7 +98,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
         {
             var metadata = ExpressionMetadataProvider.FromLambdaExpression<TModelItem, TValue>(
                 expression,
-                new ViewDataDictionary<TModelItem>(MetadataProvider),
+                new ViewDataDictionary<TModelItem>(ViewData, model: null),
                 MetadataProvider);
 
             var expressionText = ExpressionHelper.GetExpressionText(expression);

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

@@ -63,7 +63,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 return string.Empty;
             }
 
-            var viewData = new ViewDataDictionary(_viewData)
+            var viewData = new ViewDataDictionary(_viewData, model: null)
             {
                 Model = _metadata.Model,
                 ModelMetadata = _metadata

src/Microsoft.AspNet.Mvc.Core/ViewComponents/DefaultViewComponentActivator.cs

@@ -57,10 +57,7 @@ namespace Microsoft.AspNet.Mvc
                     typeof(ViewDataDictionary),
                     (context) =>
                     {
-                        return new ViewDataDictionary(context.ViewData)
-                        {
-                            Model = null
-                        };
+                        return new ViewDataDictionary(context.ViewData);
                     }
                 }
             };

src/Microsoft.AspNet.Mvc.Core/ViewComponents/ViewComponent.cs

@@ -65,12 +65,7 @@ namespace Microsoft.AspNet.Mvc
 
         public ViewViewComponentResult View<TModel>(string viewName, TModel model)
         {
-            var viewData = new ViewDataDictionary<TModel>(ViewData);
-            if (model != null)
-            {
-                viewData.Model = model;
-            }
-
+            var viewData = new ViewDataDictionary<TModel>(ViewData, model);
             return new ViewViewComponentResult(ViewEngine, viewName, viewData);
         }
     }

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

@@ -7,12 +7,6 @@ namespace Microsoft.AspNet.Mvc
 {
     public class ViewDataDictionary<TModel> : ViewDataDictionary
     {
-        /// <inheritdoc />
-        public ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider)
-            : base(metadataProvider, declaredModelType: typeof(TModel))
-        {
-        }
-
         // References may not show up due to ITypeActivator use in RazorPageActivator.
         /// <summary>
         /// Initializes a new instance of the <see cref="ViewDataDictionary{TModel}"/> class.
@@ -75,6 +69,16 @@ namespace Microsoft.AspNet.Mvc
         {
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ViewDataDictionary{TModel}"/> class.
+        /// </summary>
+        /// <remarks>Internal for testing.</remarks>
+        /// <inheritdoc />
+        internal ViewDataDictionary([NotNull] IModelMetadataProvider metadataProvider)
+            : base(metadataProvider, declaredModelType: typeof(TModel))
+        {
+        }
+
         public new TModel Model
         {
             get

src/Microsoft.AspNet.Mvc.Razor/RazorPageActivator.cs

@@ -47,13 +47,14 @@ namespace Microsoft.AspNet.Mvc.Razor
             // ViewContext.ViewData is an incompatibile type.
             if (context.ViewData == null)
             {
-                // Create ViewDataDictionary<TModel>(metadataProvider);
+                // Create ViewDataDictionary<TModel>(IModelMetadataProvider, ModelStateDictionary).
                 return (ViewDataDictionary)_typeActivator.CreateInstance(context.HttpContext.RequestServices,
-                                                                         activationInfo.ViewDataDictionaryType);
+                                                                         activationInfo.ViewDataDictionaryType,
+                                                                         context.ModelState);
             }
             else if (context.ViewData.GetType() != activationInfo.ViewDataDictionaryType)
             {
-                // Create ViewDataDictionary<TModel>(ViewDataDictionary);
+                // Create ViewDataDictionary<TModel>(ViewDataDictionary).
                 return (ViewDataDictionary)_typeActivator.CreateInstance(context.HttpContext.RequestServices,
                                                                          activationInfo.ViewDataDictionaryType,
                                                                          context.ViewData);