Don't await a DispatcherOperation

[randymarsh77]

We never cloned the awaitable duck-typing interface of DispatcherOperation in our port of it to Mac. Writing code that awaits a DispatcherOperation will compile, and then crash at runtime when executed on Mac.

Furthermore, a common mechanism of obtaining a DispatcherOperation is from a call to DispatcherUtility.BeginInvokeIfNecessary, which will return null if the invoke is not necessary. This makes would-be usage somewhat a pit of failure.

Instead, either:

• Use BeginInvoke(IfNecessary) and ignore the result if fire-and-forget is sufficient.
• Use an explicit thread-switching mechanism if an AsyncMethodContext or AsyncActions context is available.
• Use Invoke to synchronously execute code on the Dispatcher's context in a cross-platform compatible way.