Unused parameters interact poorly with inference_failure_on_untyped_parameter

Sometimes you write a function that needs to take a parameter, but doesn't intend to use that parameter at all. Sometimes that function is passed in an untyped context—this is especially common when doing JS interop, because JS is untyped *and* often passes extra parameters as "extra data" (because JavaScript doesn't care if a function takes fewer parameters than are actually passed).

I've been dealing with this using the common cross-language idiom of declaring unused parameters as `_` (or `__`/`___`/etc if there are multiples). Upon upgrading to Dart 2.6, though, this started producing a bunch of annoying `inference_failure_on_untyped_parameter` warnings, because these parameters' types couldn't be inferred. I can write `void` before them, but it's annoying to have to do so much extra work for each of these parameters when I'm not even using them.

I propose that Dart treat parameters named `_` specially, by not introducing them as names to the current scope and having their type default to `void` rather than `dynamic`. This would enforce the convention that these parameters cannot be used, and it would allow a function to declare multiple unused parameters all named `_` rather than having to add additional underscores to each one. It would also avoid these annoying warnings.