ActionScript 3 Error #1017: The definition of base class ImportedClass was not found.

You may get this error from importing a custom class that does not have the proper package path into a parent class. Another reason you will get this error is if you forget to import the Class that the subclass is extending(see example 2).

Bad Code 1
(subClass called ImportedClass)

package
{
import flash.display.MovieClip;
public class ImportedClass extends MovieClip
{
public function ImportingClass()
{
trace("Message");}}}

Parent Class called ImportingClass

package com.cjm
{
import flash.display.MovieClip;
import ImportedClass;
public class ImportingClass extends MovieClip
{
public function ImportingClass()
{
trace("Message");}}}