explodes if Object deep frozen: Object.keys polyfill modifies global Object.keys

[kumavis]

When running in strict mode within a context with deep-frozen primordials, things like Object.keys cannot be modified. This is the situation when running in Secure EcmaScript (SES) containers

in lib/keys.js

exports = module.exports = typeof Object.keys === 'function'
  ? Object.keys : shim;

exports.shim = shim;
function shim (obj) {
  var keys = [];
  for (var key in obj) keys.push(key);
  return keys;
}

if Obect.keys is present, it will have shim appended to it. if Object.keys is frozen, this will throw an Error.

node -p "'use strict'; Object.freeze(Object.keys); Object.keys.shim = () => {};"                 
[eval]:1
'use strict'; Object.freeze(Object.keys); Object.keys.shim = () => {};
                                                           ^

TypeError: Cannot add property shim, object is not extensible
    at [eval]:1:60
    at Script.runInThisContext (vm.js:122:20)
    at Object.runInThisContext (vm.js:329:38)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at Module._compile (internal/modules/cjs/loader.js:776:30)
    at evalScript (internal/bootstrap/node.js:589:27)
    at startup (internal/bootstrap/node.js:265:9)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)