buffer: allow ArrayBuffer as Buffer argument

Buffer now uses the ArrayBuffer as the backing store if passed to the
constructor.

Fixes: https://github.com/nodejs/io.js/issues/106
PR-URL: https://github.com/nodejs/io.js/pull/2002
Reviewed-By: Domenic Denicola <d@domenic.me>

test/parallel/test-buffer-arraybuffer.js

@@ -0,0 +1,46 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+const Buffer = require('buffer').Buffer;
+const LENGTH = 16;
+
+const ab = new ArrayBuffer(LENGTH);
+const dv = new DataView(ab);
+const ui = new Uint8Array(ab);
+const buf = new Buffer(ab);
+
+
+assert.ok(buf instanceof Buffer);
+// For backwards compatibility of old .parent property test that if buf is not
+// a slice then .parent should be undefined.
+assert.equal(buf.parent, undefined);
+assert.equal(buf.buffer, ab);
+assert.equal(buf.length, ab.byteLength);
+
+
+buf.fill(0xC);
+for (let i = 0; i < LENGTH; i++) {
+  assert.equal(ui[i], 0xC);
+  ui[i] = 0xF;
+  assert.equal(buf[i], 0xF);
+}
+
+buf.writeUInt32LE(0xF00, 0);
+buf.writeUInt32BE(0xB47, 4);
+buf.writeDoubleLE(3.1415, 8);
+
+assert.equal(dv.getUint32(0, true), 0xF00);
+assert.equal(dv.getUint32(4), 0xB47);
+assert.equal(dv.getFloat64(8, true), 3.1415);
+
+
+// Now test protecting users from doing stupid things
+
+assert.throws(function() {
+  function AB() { }
+  AB.__proto__ = ArrayBuffer;
+  AB.prototype.__proto__ = ArrayBuffer.prototype;
+  new Buffer(new AB());
+}, TypeError);