Test for the difference between toFixed and toString

This test comes from a note in the specification under toFixed.
It fails in ChakraCore and passes in other engines that I tested.

test/built-ins/Number/prototype/toFixed/exactness.js

@@ -0,0 +1,19 @@
+// Copyright (C) 2017 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-number.prototype.tofixed
+description: Number.prototype.toFixed does not use ToString's cleaner rounding
+info: >
+  Number.prototype.toFixed ( fractionDigits )
+
+  ...
+  8. Else x < 10^21,
+    a. Let n be an integer for which the exact mathematical value of n ÷ 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
+    b. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
+  ...
+---*/
+
+// Test from a note in the specification
+assert.sameValue((1000000000000000128).toString(0), "1000000000000000100");
+assert.sameValue((1000000000000000128).toFixed(0), "1000000000000000128");