Hide JsonDocumentTransformer (dart-lang/stream_channel#43)

Closes dart-lang/stream_channel#34

This was only used through the top level variable `jsonDocument` and no
use cases have surfaced for being able to pass a `reviver` or
`toEncodable` callback.

This implementation does not use `StreamChannelTransformer.fromCodec`
with a `JsonCodec` since the encoder and decoder are not 1:1.

- Remove the `JsonCodec` field and use `jsonEncode` and `jsonDecode`
  calls directly.
- Move the detailed doc to `jsonDocument`.
- Make the class private.
- Remove the tests constructing the class directly.
3 files changed
tree: 96f1596cf5d28311a90e89b5f16ac22dc7bc02cd
  1. pkgs/