FileReader#readAsText fails with multi-byte UTF-8 characters

`FileReader#readAsText` reads the file in chunks of 256KB. If the file contains a multi-byte UTF-8 character that is split into two separate chunks, reading fails with an encoding error (ENCODING_ERR: 5).

For many apps this is not an issue. However, if I file is larger than 256KB and contains many multi-byte characters, this is likely to happen.

I have not experienced this issue on Android yet.

Code that demonstrates the issue: https://gist.github.com/anonymous/0fdc1ec212be1e29309820477257a0c3

In the example, the reading will split the '\u0153' character into '...\x01' and '\x53', which fails to decode in UTF-8.

A workaround is to use readAsArrayBuffer instead, and do the decoding in JavaScript. However, the decoding can be quite slow on iOS where a native TextDecoder is not available.

One solution would be to make the chunk sizes semi-flexible, to ensure that it ends on a character boundary (make the chunk larger until decoding succeeds).