stream: remove legacy prependListener fallback

The prependListener method has been available on EventEmitter since
Node.js v6.0.0 (April 2016). The fallback code that manipulated the
internal _events object is no longer necessary.

This also removes tests that explicitly tested the fallback behavior.

Author: karan-lrn

lib/internal/streams/legacy.js

@@ -106,21 +106,7 @@ Stream.prototype.eventNames = function eventNames() {
 };
 
 function prependListener(emitter, event, fn) {
-  // Sadly this is not cacheable as some libraries bundle their own
-  // event emitter implementation with them.
-  if (typeof emitter.prependListener === 'function')
-    return emitter.prependListener(event, fn);
-
-  // This is a hack to make sure that our error handler is attached before any
-  // userland ones.  NEVER DO THIS. This is here only because this code needs
-  // to continue to work with older versions of Node.js that do not include
-  // the prependListener() method. The goal is to eventually remove this hack.
-  if (!emitter._events || !emitter._events[event])
-    emitter.on(event, fn);
-  else if (ArrayIsArray(emitter._events[event]))
-    emitter._events[event].unshift(fn);
-  else
-    emitter._events[event] = [fn, emitter._events[event]];
+  emitter.prependListener(event, fn);
 }
 
 module.exports = { Stream, prependListener };

test/parallel/test-event-emitter-prepend.js

@@ -17,26 +17,3 @@ myEE.prependOnceListener('foo',
                          common.mustCall(() => assert.strictEqual(m++, 0)));
 
 myEE.emit('foo');
-
-// Test fallback if prependListener is undefined.
-const stream = require('stream');
-
-delete EventEmitter.prototype.prependListener;
-
-function Writable() {
-  this.writable = true;
-  stream.Stream.call(this);
-}
-Object.setPrototypeOf(Writable.prototype, stream.Stream.prototype);
-Object.setPrototypeOf(Writable, stream.Stream);
-
-function Readable() {
-  this.readable = true;
-  stream.Stream.call(this);
-}
-Object.setPrototypeOf(Readable.prototype, stream.Stream.prototype);
-Object.setPrototypeOf(Readable, stream.Stream);
-
-const w = new Writable();
-const r = new Readable();
-r.pipe(w);

test/parallel/test-stream-events-prepend.js

@@ -2,12 +2,9 @@
 const common = require('../common');
 const stream = require('stream');
 
-class Writable extends stream.Writable {
-  constructor() {
-    super();
-    this.prependListener = undefined;
-  }
+// Test that pipe() correctly uses prependListener for error handlers.
 
+class Writable extends stream.Writable {
   _write(chunk, end, cb) {
     cb();
   }