stream: fix todo

With the update to v8 6.1 this todo can now be addressed.

PR-URL: #15667
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_readable.js

@@ -26,9 +26,7 @@ Readable.ReadableState = ReadableState;
 
 const EE = require('events');
 const Stream = require('stream');
-// TODO(bmeurer): Change this back to const once hole checks are
-// properly optimized away early in Ignition+TurboFan.
-var Buffer = require('buffer').Buffer;
+const Buffer = require('buffer').Buffer;
 const util = require('util');
 const debug = util.debuglog('stream');
 const BufferList = require('internal/streams/BufferList');