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;
			};
		}
	}( ) );
}

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