I wrote a test and found that BufferTest::testWritingToClosedStream() have the same logic I wrote.
I decide to not duplicate it. But it is required to make some changes in the code.

When $sent = fwrite($this->stream, $this->data); executes we need to check errors.
Better solution is to check errno. set_error_handler always handle error if one occurred.

if ($this->lastError['number'] > 0) {

In this case, that part of code is not needed anymore:

if (0 === $sent && feof($this->stream)) {
     $this->emit('error', array(new \RuntimeException('Tried to write to closed stream.'), $this));
     return;
}

...because in test testWritingToClosedStream() this type of error is catched by set_error_handler().
Error message contains fwrite(): send of 3 bytes failed with errno=32 Broken pipe, errno=8.

To be sure I replaced part of code I mentioned above by checking $sent === false

if ($sent === false) {
    $this->emit('error', array(new \RuntimeException('Send failed'), $this));
    return;
}

In conclusion error checking looks like that:

if ($this->lastError['number'] > 0) {
    $this->emit('error', array(
        new \ErrorException(
            $this->lastError['message'],
            0,
            $this->lastError['number'],
            $this->lastError['file'],
            $this->lastError['line']
        ),
        $this
    ));
    return;
}
if ($sent === false) {
    $this->emit('error', array(new \RuntimeException('Send failed'), $this));
    return;
}

and testWritingToClosedStream asserion looks like that:

$this->assertSame('fwrite(): send of 3 bytes failed with errno=32 Broken pipe', $error->getMessage());

All 66 tests have executed successfully.