test: writable stream finished state

Add a test for _writableState.finished.

PR-URL: https://github.com/nodejs/node/pull/8791
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Related: https://github.com/nodejs/node/issues/8686

test/parallel/test-stream-writable-finished-state.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const common = require('../common');
+
+const assert = require('assert');
+const stream = require('stream');
+
+const writable = new stream.Writable();
+
+writable._write = (chunk, encoding, cb) => {
+  // The state finished should start in false.
+  assert.strictEqual(writable._writableState.finished, false);
+  cb();
+};
+
+writable.on('finish', common.mustCall(() => {
+  assert.strictEqual(writable._writableState.finished, true);
+}));
+
+writable.end('testing finished state', common.mustCall(() => {
+  assert.strictEqual(writable._writableState.finished, true);
+}));