Style tweaks in stream_queue.dart (#167)
Style tweaks in stream_queue.dart (#167)

- Use a noun phrase to document the `hasNext` getter. The `next` and
  `peek` getters keep their verb phrase doc comments since the side
  effects are critical distinctions between these methods. If we were
  writing this today, `next()` would be a method.
- Change `_failClosed()` which returns an error, to `_checkNotClosed()`
  which checks the condition and optionally closes. Avoid nesting the
  majority of method behavior in a conditional.
- Use `RangeError.checkNotNegative` over a conditional.
- Change bare `Future` to `Future<void>` in code examples.
- Make `withTransaction` `async` since there is no longer a blocker now
  that async methods start running synchronously.
dart-lang/async