Make Flush work without requiring curly braces

Fixes #1547

samples/MvcSample.Web/Views/Home/FlushPoint.cshtml

@@ -45,8 +45,8 @@
     <strong>Marketing:</strong> <a href="mailto:Marketing@example.com">Marketing@example.com</a>
 </address>
 
+@await FlushAsync()
 @{
-    await FlushAsync();
     await Task.Delay(TimeSpan.FromSeconds(1));
 }
 <div class="row">

samples/MvcSample.Web/Views/Shared/_FlushPointLayout.cshtml

@@ -24,7 +24,7 @@
         </div>
     </div>
     <div class="container body-content">
-        @{ await FlushAsync(); }
+        @await FlushAsync()
         @RenderBody()
         @RenderSection("content")
         <hr />

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

@@ -544,8 +544,11 @@ namespace Microsoft.AspNet.Mvc.Razor
         /// Invokes <see cref="TextWriter.FlushAsync"/> on <see cref="Output"/> writing out any buffered
         /// content to the <see cref="HttpResponse.Body"/>.
         /// </summary>
-        /// <returns>A <see cref="Task"/> that represents the asynchronous flush operation.</returns>
-        public Task FlushAsync()
+        /// <returns>A<see cref="Task{HtmlString}"/> that represents the asynchronous flush operation and on
+        /// completion returns a <see cref="HtmlString.Empty"/>.</returns>
+        /// <remarks>The value returned is a token value that allows FlushAsync to succeed. However the
+        /// value does not represent the rendered content.</remarks>
+        public async Task<HtmlString> FlushAsync()
         {
             // If there are active writing scopes then we should throw. Cannot flush content that has the potential to
             // change.
@@ -562,7 +565,8 @@ namespace Microsoft.AspNet.Mvc.Razor
                 throw new InvalidOperationException(message);
             }
 
-            return Output.FlushAsync();
+            await Output.FlushAsync();
+            return HtmlString.Empty;
         }
 
         /// <inheritdoc />

test/Microsoft.AspNet.Mvc.Razor.Test/RazorPageTest.cs

@@ -98,10 +98,10 @@ namespace Microsoft.AspNet.Mvc.Razor
         {
             // Arrange
             var viewContext = CreateViewContext();
-            var page = CreatePage(v =>
+            var page = CreatePage(async v =>
             {
                 v.StartWritingScope();
-                v.FlushAsync();
+                await v.FlushAsync();
             });
 
             // Act
@@ -527,6 +527,25 @@ namespace Microsoft.AspNet.Mvc.Razor
             await Assert.DoesNotThrowAsync(() => renderAsyncDelegate(TextWriter.Null));
         }
 
+        [Fact]
+        public async Task FlushAsync_ReturnsEmptyHtmlString()
+        {
+            // Arrange
+            HtmlString actual = null;
+            var writer = new Mock<TextWriter>();
+            var context = CreateViewContext(writer.Object);
+            var page = CreatePage(async p =>
+            {
+                actual = await p.FlushAsync();
+            }, context);
+
+            // Act
+            await page.ExecuteAsync();
+
+            // Assert
+            Assert.Same(HtmlString.Empty, actual);
+        }
+
         [Fact]
         public async Task WriteAttribute_CallsBeginAndEndContext_OnPageExecutionListenerContext()
         {

test/WebSites/RazorWebSite/Views/FlushPoint/PageWithoutLayout.cshtml

@@ -1,12 +1,12 @@
 @inject WaitService WaitService
 Initial content
+@await FlushAsync()
 @{
-    await FlushAsync();
     WaitService.WaitForClient();
 }
 Secondary content
+@await FlushAsync()
 @{
-    await FlushAsync();
     WaitService.WaitForClient();
 }
 Final content

test/WebSites/RazorWebSite/Views/Shared/_LayoutWithFlush.cshtml

@@ -1,7 +1,7 @@
 @inject WaitService WaitService
 <title>@ViewBag.Title</title>
+@await FlushAsync()
 @{
-    await FlushAsync();
     WaitService.WaitForClient();
 }
 @RenderBody()

test/WebSites/RazorWebSite/Views/Shared/_LayoutWithPartialAndFlush.cshtml

@@ -1,8 +1,8 @@
 @inject WaitService WaitService
 <title>@ViewBag.Title</title>
 @RenderBody()
+@await FlushAsync()
 @{
-    await FlushAsync();
     WaitService.WaitForClient();
 }
 @await Html.PartialAsync("_PartialThatSetsTitle")