ディクショナリと連想配列
何度も繰り返し呼ばれる処理内では効果が出ます。
Performance Tip: Dictionaries and Associative Arrays
http://www.insideria.com/2008/04/dictionaries-and-associative-a.html
Associative arrays with string keys
http://livedocs.adobe.com/flex/3/html/help.html?content=10_Lists_of_data_4.html