Rewrite transformers as extension methods (dart-lang/stream_transform#83)

The usage is much nicer as extension methods.

In most cases a small bit of code is copied for the replacement, though
in some cases nearly the entire implementation is copied. This was done
over extracting shared pieces so that the deprecated methods can be
removed outright in the next major version change.

Update and clean up some doc comments in the extension method version of
these functions.

Migrate tests to use the extension method versions. The old copies are
now untested but they won't be modified before they are removed.

Since there aren't useful needs within this package for
`chainTransformers` or `map` they are deprecated without a replacement.
42 files changed
tree: 04413b037632b6f6ca9b77477f267419b47e4dbb
  1. pkgs/