Begin ability to have .cshtml files in Blazor apps

samples/StandaloneApp/Index.cshtml

@@ -0,0 +1,3 @@
+@inherits Microsoft.Blazor.Components.BlazorComponent
+<h1>Hello, world!</h1>
+Hello from the Razor component.

samples/StandaloneApp/Program.cs

@@ -4,7 +4,6 @@
 using Microsoft.Blazor.Browser.Rendering;
 using Microsoft.Blazor.Components;
 using Microsoft.Blazor.RenderTree;
-using System;
 
 namespace StandaloneApp
 {

src/Microsoft.Blazor/Components/BlazorComponent.cs

@@ -0,0 +1,38 @@
+// 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 Microsoft.Blazor.RenderTree;
+using System;
+using System.Threading.Tasks;
+
+namespace Microsoft.Blazor.Components
+{
+    /// <summary>
+    /// Optional base class for Blazor components. Alternatively, Blazor components may
+    /// implement <see cref="IComponent"/> directly.
+    /// </summary>
+    public abstract class BlazorComponent : IComponent
+    {
+        /// <inheritdoc />
+        public virtual void BuildRenderTree(RenderTreeBuilder builder)
+        {
+            // This is virtual rather than abstract so that 'code behind' classes don't have to
+            // be marked abstract.
+            // Developers can either override this method in derived classes, or can use Razor
+            // syntax to define a derived class and have the compiler generate the method.
+        }
+
+        // At present, if you have a .cshtml file in a project with <Project Sdk="Microsoft.NET.Sdk.Web">,
+        // Visual Studio will run design-time builds for it, codegenning a class that attempts to override
+        // this method. Therefore the virtual method must be defined, even though it won't be used at runtime,
+        // because otherwise VS will display a design-time error in its 'Error List' pane.
+        // TODO: Track down what triggers the design-time build for .cshtml files and how to stop it, then
+        // this method can be removed.
+        /// <summary>
+        /// Not used. Do not invoke this method.
+        /// </summary>
+        /// <returns>Always throws an exception.</returns>
+        public virtual Task ExecuteAsync()
+            => throw new NotImplementedException($"Blazor components do not implement {nameof(ExecuteAsync)}.");
+    }
+}

src/Microsoft.Blazor/Components/RazorToolingWorkaround.cs

@@ -0,0 +1,43 @@
+// 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.
+
+/*
+ * Currently if you have a .cshtml file in a project with <Project Sdk="Microsoft.NET.Sdk.Web">,
+ * Visual Studio will run design-time builds for the .cshtml file that assume certain ASP.NET MVC
+ * APIs exist. Since those namespaces and types wouldn't normally exist for Blazor client apps,
+ * this leads to spurious errors in the Errors List pane, even though there aren't actually any
+ * errors on build. As a workaround, we define here a minimal set of namespaces/types that satisfy
+ * the design-time build.
+ * 
+ * TODO: Track down what is triggering the unwanted design-time build and find out how to disable it.
+ * Then this file can be removed entirely.
+ */
+
+using System;
+
+namespace Microsoft.AspNetCore.Mvc
+{
+    public interface IUrlHelper { }
+    public interface IViewComponentHelper { }
+}
+
+namespace Microsoft.AspNetCore.Mvc.Razor
+{
+    public class RazorPage<T> { }
+
+    namespace Internal
+    {
+        public class RazorInjectAttributeAttribute : Attribute { }
+    }
+}
+
+namespace Microsoft.AspNetCore.Mvc.Rendering
+{
+    public interface IJsonHelper { }
+    public interface IHtmlHelper<T> { }
+}
+
+namespace Microsoft.AspNetCore.Mvc.ViewFeatures
+{
+    public interface IModelExpressionProvider { }
+}