On Windows `_NativeSocket.nativeWrite` returning to caller does
not mean that the data was flushed into OS buffer. Overlapped
operation might still be in progress - and it will be cancelled
with data loss if we close the handle.
This means _SocketStreamConsumer should take such pending writes
into account and not issue `done` event until pending write
completes.
This change prevents data loss when communicating with
subprocesses on Windows. Previously even doing
`await process.stdin.flush()` would not guarantee data
delivery.
Fixes https://github.com/dart-lang/sdk/issues/50904
TEST=tests/standalone/io/regress_50904_test.dart
Cq-Include-Trybots: luci.dart.try:vm-win-release-x64-try,vm-win-debug-x64-try,pkg-win-release-try
Change-Id: I13ab2cc3ce45f1ff8cb3183884d26bbb2b72f7ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/307043
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
