The most obtrusive and dark trick to promote objects inheritance without actually loosing the current one if not at the deepest level. It does not pollute the `Object.prototype`

promote.js

var promote = function(getPrototypeOf, setPrototypeOf){
  // (C) The Evilness
  return function(self, Class){
    var
      promote = typeof Class == 'function' ?
        Class.prototype : getPrototypeOf(Class),
      current = self,
      previous,
      next
    ;
    if (promote && !promote.isPrototypeOf(self)) {
      do {
        previous = current;                        // {} or latter prototype
        current = getPrototypeOf(current);         // Object.prototype
        next = current && getPrototypeOf(current); // null
      } while(next);
      setPrototypeOf(previous, promote);
    }
    return self;
  };
}(
  Object.getPrototypeOf ||
  function(o){return o.__proto__},
  Object.setPrototypeOf ||
  function(o,p){o.__proto__=p}
);

// example
function A() {}
function B() {}
B.prototype = new A;
function C() {}
var b = new B;
// true, true, false
alert([b instanceof A, b instanceof B, b instanceof C]);
promote(b, C);

// now true, true, true
alert([b instanceof A, b instanceof B, b instanceof C]);

// but this is true too now!!!
alert(A.prototype instanceof C);

promote(b = {}, C);
alert(b instanceof C);