Object.assign erroneous TypeError

[rwaldron]

This was exposed by @yanlxu's tests here: https://github.com/tc39/test262/pull/293/files

Ref:

es6-shim/es6-shim.js

Line 1128 in cba1442

if (!ES.TypeIsObject(target)) {

Object.assign must:

• Coerce the target to an object, per ToObject
• Throw a TypeError exception for cases that cannot be coerced, per ToObject
• Apply the same semantics when there is only a target (this is pretty specific to es6-shim, but the patch will make it clear)