Fix #974

The root cause here was that we weren't setting the language version in
MSBuild, which is only for the command line version.

(cherry picked from commit 319e31f71a150e9b0d91e724f0e358390caec4c2)

src/Microsoft.AspNetCore.Blazor.Build/targets/RazorCompilation.targets

@@ -14,6 +14,8 @@
     <!-- Deactivates the Razor SDK's build-time compilation. We do our own -->
     <RazorCompileOnBuild>false</RazorCompileOnBuild>
 
+    <RazorLangVersion>Experimental</RazorLangVersion>
+
     <!-- 
       This version is used to build our RazorConfiguration, and is hardcoded into the tools in VS. If you change
       This to a value that doesn't match VS, then the Blazor Razor experience won't work for those documents.

test/Microsoft.AspNetCore.Blazor.E2ETest/Tests/ComponentRenderingTest.cs

-// Copyright (c) .NET Foundation. All rights reserved.
+// 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 System;
@@ -40,6 +40,17 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Tests
             Assert.Equal("Hello from TextOnlyComponent", appElement.Text);
         }
 
+        // This verifies that we've correctly configured the Razor language version via MSBuild.
+        // See #974
+        [Fact]
+        public void CanRenderComponentWithDataDash()
+        {
+            var appElement = MountTestComponent<DataDashComponent>();
+            var element = appElement.FindElement(By.Id("cool_beans"));
+            Assert.Equal("17", element.GetAttribute("data-tab"));
+            Assert.Equal("17", element.Text);
+        }
+
         [Fact]
         public void CanRenderComponentWithAttributes()
         {

test/testapps/BasicTestApp/DataDashComponent.cshtml

+<div id="cool_beans" data-tab="@TabId">@TabId</div>
+
+@functions {
+    string TabId = "17";
+}