function isNativeFunction( value ) {
	return (
		isFunction( value ) &&
		reNativeFunction.REGEXP.test( function2string( value ) )
	);
}
Tests if a value is a native function.

@param {*} value - value to test
@returns {boolean} boolean indicating whether value is a native function

@example
function beep() {
    console.log( 'boop' );
}

var bool = isNativeFunction( beep );
// returns false

bool = isNativeFunction( Date );
// returns true