Undo instanceof Buffer changes

It turns out that it's not possible to use `instanceof Buffer` safely,
like I thought.

It's not possible to use `instanceof Buffer` reliably in a browserify
context because there could be multiple different copies of the
'buffer' package in use.

This previous method (checking `buf._isBuffer`) works even for Buffer
instances that were created from another copy of the `buffer` package.

NOTE: It's possible to have two different "instances" of the 'buffer'
package, even if the 'buffer' package appears only once in the bundled
code. This can happen if you require 'buffer' in different ways, for
example:

`require('buffer')` vs. `require('buffer/')` vs. using the implicit
`Buffer` global.

You can confirm this by browserifying this code:

```js
console.log(require('buffer').Buffer === require('buffer/').Buffer) //
will be false
```

So, for this reason, `instanceof` won't work.

Author: feross

index.js

@@ -239,7 +239,7 @@ function fromArrayBuffer (array, byteOffset, length) {
 }
 
 function fromObject (obj) {
-  if (obj instanceof Buffer) {
+  if (Buffer.isBuffer(obj)) {
     var len = checked(obj.length) | 0
     var buf = createBuffer(len)
 
@@ -285,11 +285,11 @@ function SlowBuffer (length) {
 }
 
 Buffer.isBuffer = function isBuffer (b) {
-  return b instanceof Buffer
+  return b != null && b._isBuffer === true
 }
 
 Buffer.compare = function compare (a, b) {
-  if (!(a instanceof Buffer) || !(b instanceof Buffer)) {
+  if (!Buffer.isBuffer(a) || !Buffer.isBuffer(b)) {
     throw new TypeError('Arguments must be Buffers')
   }
 
@@ -351,7 +351,7 @@ Buffer.concat = function concat (list, length) {
   var pos = 0
   for (i = 0; i < list.length; ++i) {
     var buf = list[i]
-    if (!(buf instanceof Buffer)) {
+    if (!Buffer.isBuffer(buf)) {
       throw new TypeError('"list" argument must be an Array of Buffers')
     }
     buf.copy(buffer, pos)
@@ -361,7 +361,7 @@ Buffer.concat = function concat (list, length) {
 }
 
 function byteLength (string, encoding) {
-  if (string instanceof Buffer) {
+  if (Buffer.isBuffer(string)) {
     return string.length
   }
   if (ArrayBuffer.isView(string) || string instanceof ArrayBuffer) {
@@ -474,8 +474,12 @@ function slowToString (encoding, start, end) {
   }
 }
 
-// The property is used by the `is-buffer` npm package to detect Buffer instances
-// in Safari 5-7. Remove this eventually.
+// This property is used by `Buffer.isBuffer` (and the `is-buffer` npm package)
+// to detect a Buffer instance. It's not possible to use `instanceof Buffer`
+// reliably in a browserify context because there could be multiple different
+// copies of the 'buffer' package in use. This method works even for Buffer
+// instances that were created from another copy of the `buffer` package.
+// See: https://github.com/feross/buffer/issues/154
 Buffer.prototype._isBuffer = true
 
 function swap (b, n, m) {
@@ -529,7 +533,7 @@ Buffer.prototype.toString = function toString () {
 }
 
 Buffer.prototype.equals = function equals (b) {
-  if (!(b instanceof Buffer)) throw new TypeError('Argument must be a Buffer')
+  if (!Buffer.isBuffer(b)) throw new TypeError('Argument must be a Buffer')
   if (this === b) return true
   return Buffer.compare(this, b) === 0
 }
@@ -545,7 +549,7 @@ Buffer.prototype.inspect = function inspect () {
 }
 
 Buffer.prototype.compare = function compare (target, start, end, thisStart, thisEnd) {
-  if (!(target instanceof Buffer)) {
+  if (!Buffer.isBuffer(target)) {
     throw new TypeError('Argument must be a Buffer')
   }
 
@@ -647,7 +651,7 @@ function bidirectionalIndexOf (buffer, val, byteOffset, encoding, dir) {
   }
 
   // Finally, search either indexOf (if dir is true) or lastIndexOf
-  if (val instanceof Buffer) {
+  if (Buffer.isBuffer(val)) {
     // Special case: looking for empty string/buffer always fails
     if (val.length === 0) {
       return -1
@@ -1213,7 +1217,7 @@ Buffer.prototype.readDoubleBE = function readDoubleBE (offset, noAssert) {
 }
 
 function checkInt (buf, value, offset, ext, max, min) {
-  if (!(buf instanceof Buffer)) throw new TypeError('"buffer" argument must be a Buffer instance')
+  if (!Buffer.isBuffer(buf)) throw new TypeError('"buffer" argument must be a Buffer instance')
   if (value > max || value < min) throw new RangeError('"value" argument is out of bounds')
   if (offset + ext > buf.length) throw new RangeError('Index out of range')
 }
@@ -1541,7 +1545,7 @@ Buffer.prototype.fill = function fill (val, start, end, encoding) {
       this[i] = val
     }
   } else {
-    var bytes = val instanceof Buffer
+    var bytes = Buffer.isBuffer(val)
       ? val
       : new Buffer(val, encoding)
     var len = bytes.length