Add placeholder DefineSection method.

- This enables 2.0.0 code generation to not error.

aspnet/Razor#1254

src/Microsoft.AspNetCore.Mvc.Razor/RazorPageBase.cs

@@ -292,6 +292,17 @@ public virtual string Href(string contentPath)
             return _urlHelper.Content(contentPath);
         }
 
+        /// <summary>
+        /// Creates a named content section in the page that can be invoked in a Layout page using
+        /// <c>RenderSection</c> or <c>RenderSectionAsync</c>
+        /// </summary>
+        /// <param name="name">The name of the section to create.</param>
+        /// <param name="section">The delegate to execute when rendering the section.</param>
+        /// <remarks>This is a temporary placeholder method to support ASP.NET Core 2.0.0 editor code generation.</remarks>
+        [EditorBrowsable(EditorBrowsableState.Never)]
+        protected void DefineSection(string name, Func<object, Task> section)
+            => DefineSection(name, () => section(null /* writer */));
+
         /// <summary>
         /// Creates a named content section in the page that can be invoked in a Layout page using
         /// <c>RenderSection</c> or <c>RenderSectionAsync</c>