Work around global Object.prototype pollution

[jcready]

Fixes #724

There shouldn’t be any issue in mutating the baseDescriptors object directly since:

1. Object.getOwnPropertyDescriptors() always returns a referentially unique object every time it’s called.
2. Object.create() doesn’t retain any references to the second argument (the descriptors) in the resulting object.

var a = Object.getPrototypeOf({});
var b = Object.getPrototypeOf({});
a === b; // true

var descriptor  = Object.getOwnPropertyDescriptors(a);
var descriptor2 = Object.getOwnPropertyDescriptors(a);
descriptor === descriptor2; // false

descriptor.field = {};

descriptor.field.value = 1;
var one = Object.create(null, descriptor);
one.field === 1; // true

descriptor.field.value = 2;
var two = Object.create(null, descriptor);
two.field === 2; // true
one.field === 1; // still true

[jcready]

Assuming this lands I predict the next bug related to this will be along the lines of "It looks like [name of widely used package] modifies Object.getOwnPropertyDescriptors() to always return a frozen object so assigning a property to the resulting object like this throws an error."

[timostamm]

Thanks. Pushed up 2db9c80 to satisfy type checks.