fix(neon): Fix SVG code compilation for web

[provokateurin]

Extracted from #372
This wasn't compiling before because it was relying on dart:io code internally.

[Leptopoda]

From the SvgLoader documentation:

/// A [BytesLoader] that parses a SVG data in an isolate and creates a
/// vector_graphics binary representation.

This means loading the file could happen in a separate isolate thus not blocking the main one.
I'd rather make a custom UniversalSvgFileLoader as each loader is only around 30 lines of code.

[provokateurin]

The annoying thing is that this code isn't even executed on web since we have no push notification support there. Maybe we should instead use a stub on web to fix the compilation (conditional import if you know what I mean).

[Leptopoda]

I don't know what makes most sense rn but this shouldn't even be a problem with #808.
Do you think it's worth stubbing?

[provokateurin]

I think I will go with the stub for now and once we have the plugin system we can easily drop it.

[provokateurin]

I went with your suggestion because creating the loader was very easy indeed. Just had to copy the SvgFileLoader and replace dart:io with universal_io.