[generator] Emit new when hiding Handle, Message

[jonpryor]

The internal Xamarin.Android CI system is reporting an error when
running the generator unit tests, but it's a false positive.
It's not a bug in generator. Rather, it's a bug in Mono 4.2, in
which CSharpCodeCompiler treats multi-line warnings as errors.
Since the generator unit tests emit such multi-line warnings when
hiding Java.Lang.Object.Handle (via
Xamarin.Test.SomeObject.Handle()) and System.Exception.Message
(via Java.Lang.Throwable.Message), and Mono 4.2 treats the
multi-line warnings as errors, the tests fail.

There are three plausible solutions so that we don't get erroneous
reports from the generator tests when running on Mono 4.2:

1. Remove those tests. (Uh...no.)
2. Upgrade Mono on the CI machine to Mono 4.4, which has the fix.
3. Fix generator so that the warning isn't emitted.

(2) would require an unknown timeframe with unknown repercussions.

Thus, the chosen fix: improve generator so that the warning isn't
generated.

[atsushieno]

Your fix is not going to change the situation with respect to this, but shouldn't Message override Exception.Message instead of declaring as new?

I'm also quite unsure why Handle is generated like that, shouldn't that rather be eliminated?

[jonpryor]

Message is new and not override because of java.lang.Throwable.getMessage(), and (mostly) the generator doesn't know about Exception.Message.

So it was easier to make it new.

Additionally, Throwable.Message is already (implicitly) new. I don't know if it would be a breaking change to change it to override. :-(

I'm also quite unsure why Handle is generated like that, shouldn't that rather be eliminated?

I don't understand. Do you mean SomeObject.Handle()? If so, that was to test the qualification of the Object.Handle property, so that new Handle() members could be introduced without breaking things.