Throw InvalidOperationException instead of silently ignoring an invalid HostObject

src/Compilers/Core/MSBuildTask/Csc.cs

@@ -206,6 +206,10 @@ protected internal override void AddResponseFileCommands(CommandLineBuilderExten
             {
                 designTime = csHost.IsDesignTime();
             }
+            else if (HostObject != null)
+            {
+                throw new InvalidOperationException();
+            }
             if (!designTime)
             {
                 if (!string.IsNullOrWhiteSpace(VsSessionGuid))

src/Compilers/Core/MSBuildTask/Vbc.cs

@@ -510,6 +510,10 @@ protected internal override void AddResponseFileCommands(CommandLineBuilderExten
             {
                 designTime = vbHost.IsDesignTime();
             }
+            else if (this.HostObject != null)
+            {
+                throw new InvalidOperationException();
+            }
             if (!designTime)
             {
                 if (!string.IsNullOrWhiteSpace(this.VsSessionGuid))