Fix calling endOfStream on a MockStreamHandlerEventSink causing widget tests to hang - #190808

…r#196110)

Fixes flutter#190807

This continues flutter#190808 by @SamJakob, which was approved and then closed
by the bot while it waited for a reply. It has the same fix, with the
review comments on flutter#190808 applied.

`TestDefaultBinaryMessenger.setMockStreamHandler` adds
`controller.close` as a tear down. If the handler already closed the
stream, for example with `MockStreamHandlerEventSink.endOfStream` in a
`testWidgets` body, tear down calls `close()` again and waits for the
returned future. Tear down then does not finish, and the test times out.

The fix closes the controller in tear down only if it is not closed.

About the question on flutter#190808: yes, the future completes. The zone is
the cause. I checked this with a temporary probe in the tear down (not
part of this PR):
- The done future of the controller completes during the `testWidgets`
body, in the fake async zone.
- In tear down, `isClosed` is true, and `close()` returns the same done
future.
- A `then` callback that tear down adds to this future does not run,
also after 2 seconds of real time.

A completed `_Future` schedules its listeners as microtasks in its own
zone (`_addListener` in `future_impl.dart`). That zone is the fake async
zone, and nothing flushes its microtasks after the body ends. So tear
down waits forever. The comment on the guard says this.

On the other review comments on flutter#190808:
- The guard has a comment that says why it is necessary.
- The test uses a 5 second timeout (not 1 second), so that a hang fails
fast and a slow bot does not make the test flaky.

Tests:
- New regression test in `test_default_binary_messenger_test.dart`.
Without the fix it fails with `TimeoutException after 0:00:05`. With the
fix it passes.
- `packages/flutter_test` tests and `packages/flutter/test/services`
tests pass.


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---------

Co-authored-by: SamJakob <[email protected]>