code: @BackpressureSupport(BackpressureKind.FULL)
      @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public static <T> Flowable<T> concatEager(Publisher<? extends SingleSource<? extends T>> sources) {
          return Flowable.fromPublisher(sources).concatMapEager(SingleInternalHelper.<T>toFlowable());
      }
docs: Concatenates a Publisher sequence of SingleSources eagerly into a single stream of values.
  <p>
  <img width="640" height="307" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.concatEager.p.png" alt="">
  <p>
  Eager concatenation means that once a subscriber subscribes, this operator subscribes to all of the
  emitted source Publishers as they are observed. The operator buffers the values emitted by these
  Publishers and then drains them in order, each one after the previous one completes.
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>Backpressure is honored towards the downstream and the outer Publisher is
  expected to support backpressure. Violating this assumption, the operator will
  signal {@link io.reactivex.exceptions.MissingBackpressureException}.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>This method does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <T> the value type
  @param sources a sequence of Publishers that need to be eagerly concatenated
  @return the new Publisher instance with the specified concatenation behavior