test: writable stream needDrain state

Add a test for _writableState.needDrain.

PR-URL: https://github.com/nodejs/node/pull/8799
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Related: https://github.com/nodejs/node/issues/8686

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

@@ -0,0 +1,23 @@
+'use strict';
+
+const common = require('../common');
+const stream = require('stream');
+const assert = require('assert');
+
+const transform = new stream.Transform({
+  transform: _transform,
+  highWaterMark: 1
+});
+
+function _transform(chunk, encoding, cb) {
+  assert.strictEqual(transform._writableState.needDrain, true);
+  cb();
+}
+
+assert.strictEqual(transform._writableState.needDrain, false);
+
+transform.write('asdasd', common.mustCall(() => {
+  assert.strictEqual(transform._writableState.needDrain, false);
+}));
+
+assert.strictEqual(transform._writableState.needDrain, true);