Polyfill for Object.getPrototypeOf -- It will work unless your constructor's prototype doesn't have a constructor property (which means you assigned another object as prototype and didn't give it a constructor property...)

Object.getPrototypeOf.js

if ( typeof Object.getPrototypeOf !== "function" ) {
	( function ( ) {
		function getPrototypeValue( o, p ) {
			if ( o.hasOwnProperty( p ) ) {
				var ownValue = o[ p ];
				if ( delete o[ p ] ) {
					var prototypeValue = o[ p ];
					o[ p ] = ownValue;
					return prototypeValue;
				} else {
					return o[ p ];
				}
			} else {
				return o[ p ];
			}
		}
		if ( typeof "".__proto__ === "object" ) {
			Object.getPrototypeOf = function( object ) {
				return getPrototypeValue( object, '__proto__' );
			};
		} else {
			Object.getPrototypeOf = function( object ) {
				getPrototypeValue( object, 'constructor' ).prototype;
			};
		}
	}( ) );
}

It needs the constructor property to work properly.

What makes this work consistently is to enforce it with an Object.create polyfill that adds proto to all newly created objects when called, that way your else never gets used, but I suppose I am digressing...

When you say proto you mean proto ? Because there are some browser supproting proto and not Object.create. So it would create some weird bugs.
And if it is proto then you need to add an else if.

if you polyfill Object.create to add the proto then you won't hit the else above.

You'd still need it to work on objects created with the new operator.