After this patch, Flutter apps on Fuchsia can access their asset
bundles. Unlike other platforms, the asset bundles are stored in memory
rather than in the file system.
After this patch, we're able to run spinning_square on Fuchsia. I've
left some TODO comments about some shortcuts I've taken that we'll need
to improve in the future.
This patch should contain all the code we need to run Flutter on
Fuchsia's software framebuffer (without text). I haven't actually tried
running the code, so I'm sure it doesn't work yet.
