getBufferSubDataAsync 方法
实现
Future getBufferSubDataAsync(int target, int srcByteOffset, TypedData dstData,
[int? dstOffset, int? length]) =>
promiseToFuture(JS("", "#.getBufferSubDataAsync(#, #, #, #, #)", this,
target, srcByteOffset, dstData, dstOffset, length));