sampleOn for Flyd.
Samples from the second stream every time an event occurs on the first stream.
Signature
Stream a -> Stream b -> Stream b
Usage
// Assume `sendBtnClicked` emits whenever a send button is pressed and
// `messageText` is a stream of the current content of an input field.
// Then `sendMessage` emits the content of the text field whenever the button
// is pressed.
var sendMessage = sampleOn(sendBtnClicked, messageText);