package
{
	import flash.display.Sprite;
	import flash.utils.getDefinitionByName;

	public class DynamicCall extends Sprite
	{
		public function DynamicCall()
		{
            var ClassReference:Class = getDefinitionByName("String") as Class;
            var s:String = (new ClassReference("foo=") as String);
            trace(s);
		}
	}
}

// This basically creates an instance of the String class, from the 
// class name 'String'. getDefinitionByName takes the entire class 
// path, so if you wanted to create an instance of MovieClip, you would 
// provide the entire path:
// var ClassReference:Class = getDefinitionByName("flash.display.MovieClip") as Class;

http://www.mikechambers.com/blog/2006/06/22/actionscript-3-get-a-class-reference-by-class-name/

AS3 Get a Class Reference by Class Name

class

ActionScript 3