Merge pull request dart-lang#83 from dart-lang/config_specific_imports

Add `WebSocketChannel.connect` factory constructor.

Use `drain` to reach end of stream in tests (dart-lang#75)

Replace `.toList()` for cases where we don't care about the collected
values.

Replace `.listen(null).asFuture()` since that is exactly the
implementation of `drain`.

Declare a return value of `Uint8List` in `BytesBuilder` methods (dart…

…-lang#58)

Part of the rollout of dart-lang/sdk#36900

Allow stream_channel version 2.x (dart-lang#48)

None of the breaking changes impact this package or it's exposed API.

Update Dart SDK dependency.

Support the latest dev SDK.