Enable unstable APIs for stable types.

[halzy]

When unstable types are used in stable methods, those methods should be marked as unstable.

For example:
ReadableStream::pipe_to(writer: WritableStream) (WritableStream is unstable)

ReadableWritablePair is a dictionary of a ReadableStream and WritableStream. It's constructor and setter for the WritableStream should be unstable.

[alexcrichton]

Thanks!