Use case

Many platforms, Android included, may store or manage files using non-standard File APIs. In some cases these may map nicely to existing APIs (for ex Win32), but in some cases these may not fit as well (for ex Android NDK AAsset, HTTPS, FTP, etc).

Proposal

To allow Flutter to become more portable for custom embedders without a standard filesystem, I propose the following changes:

• Expose AssetResolver through Embedder API (Embedder Asset Provider engine#29802)
• Allow PersistentCache to be overridden/provided by the Embedder
• Add GN configuration options for linking with Dart without dart:io
• Add C define (tentatively FLUTTER_NO_IO) to ifdef out I/O dependent parts of Engine and FML

This should allow builds of Flutter with embedder-provided I/O. Of course, deeper changes are needed in dependencies like Dart when the target platform is "barely POSIX" but this is out of scope for this issue.