readAsArrayBuffer polyfill for Firefox 4 with caching

mozilla-readasarraybuffer.js

if (!FileReader.prototype.readAsArrayBuffer) {
	FileReader.prototype.readAsArrayBuffer = function readAsArrayBuffer () {
		this.readAsBinaryString.apply(this, arguments);
		this.__defineGetter__('resultString', this.__lookupGetter__('result'));
		Object.defineProperty(this, 'result', {
			get : function () {
				var string = this.resultString,
					result = new Uint8Array(string.length);
				for (var i = 0; i < string.length; i++) {
					result[i] = string.charCodeAt(i);
				}
				//only incur the cost of copying once!
				this.__defineGetter__('result', function(){
					return result.buffer;
				});
				return result.buffer;
			},configurable:true
		});
	};
}