Set 'NoRestartServerOnBuild' by default. This means we can re-enable …

…IISExpress when we think people are all on VS15.7Pre3 or later

src/Microsoft.AspNetCore.Blazor.Build/targets/All.props

@@ -7,6 +7,9 @@
     <!-- By default, enable auto rebuilds for debug builds. Note that the server will not enable it in production environments regardless. -->
     <BlazorRebuildOnFileChange Condition="'$(Configuration)' == 'Debug'">true</BlazorRebuildOnFileChange>
 
+    <!-- When using IISExpress with a standalone app, there's no point restarting IISExpress after build. It slows things unnecessarily and breaks in-flight HTTP requests. -->
+    <NoRestartServerOnBuild>true</NoRestartServerOnBuild>
+
     <!-- We can remove this after updating to newer Razor tooling, where it's enabled by default -->
     <UseRazorBuildServer>true</UseRazorBuildServer>
   </PropertyGroup>