Demonstrate Dart calling into a .NET component via a CCW

[timsneath]

Good suggestion from @escamoteur.

Requires:

• Add support for remaining COM Callable Wrapper interfaces (https://docs.microsoft.com/en-us/dotnet/standard/native-interop/com-callable-wrapper)
• Create a trivial component in C# or similar, and create a tlb for it
• Create a demo app that calls the C# component from Dart
• Test, document and publish

[timsneath]

#111 takes care of the first one of these. Now to try it out.

[RustamG]

Hey @timsneath!

Any ETA on the remaining steps?

Thanks.

[timsneath]

Afraid not. I put this down to work on some other cool Win32 package-related things, and I haven't had a chance to pick it up again. I've marked it as 'help wanted' in case there are others who have the time to look at this specifically.