get the global object as reliably as possible

getGlobal.js

var getGlobal = (function(g) {
  if (g == null) {
    if (typeof System !== 'undefined' && System != null && System.global != null && System.global.System === System) g = System.global;
    else if (typeof self !== 'undefined' && self != null && self.self === self) g = self;
    else if (typeof window !== 'undefined' && window != null && window.window === window) g = window;
    else if (typeof global !== 'undefined' && global != null && global.global === global) g = global;
  }
  return function() { return g; };
}(this));