HtmlImage.toByteData has surprising behavior

[dnfield]

It seems like we should be throwing an exception instead of returning null at https://github.com/flutter/engine/blob/e3444d895545b629370fcf0ddd43f84b361ed306/lib/web_ui/lib/src/engine/html_image_codec.dart#L137

We also ignore the format parameter. It's not really clear to me right now what format we actually return, but I suspect it's the original encoded format of the image rather than the format the caller is requesting. The caller can request raw RGBA array or request PNG data, or "rawUnmodified," which should probably be the same as rawRgba on web.

[dnfield]

From yegor:

Raw pixels: https://developer.mozilla.org/en-US/docs/Web/API/ImageData/data
PNG: https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL

[yjbanov]

As the HTML renderer has been deprecated and removed from the default Flutter SDK, we're now closing all issues that affect only the HTML renderer. For more information, please see:

• ☂️ Intent to deprecate and remove the HTML renderer in Flutter Web #145954
• [web] Actual removal of HTML code #164003