Add a shared ignoreArgument callback (dart-lang/stream_transform#190)

Add a `common_callbacks.dart` library with a single top level function
`ignoreArgument` which takes a single `dynamic` argument and does
nothing. Replace all `(_) => null` function literals, as well as a few
library-private functions with unnecessarily specific signatures, with
the shared definition.

As suggested in dart-lang/stream_transform#188