The test should not assume that a stream of bytes delivers them in a
certain chunked order (e.g. all bytes of a line are delivered in one
chunk and the newline comes in another chunk).
Instead it should use a line splitter to obtain individual lines and can
match those against expectations.
This fragile test is broken after [0] which combines writing a string
together with the newline instead of seperately.
[0] https://dart-review.googlesource.com/c/sdk/+/378024
493c453d57