THttpClient fails when using async support outside Silverlight

THttpClient fails if you are using the standard .NET framework (i.e. not Silverlight), and you have generated your proxies using the async option. The error message is pretty unhelpful:

"No request has been sent"

This is caused by the BeginFlush method not being implemented outside Silverlight, which means that the request isn't sent.

I think two changes should be made:

• Either make BeginFlush / EndFlush abstract instead of virtual, or throw a NotSupportedException if the base implementation is called so that it's obvious what's going on.
• Remove the conditional preprocessor directives that exclude the implementations of BeginFlush / EndFlush.