Add a DynamicBuffer for faster VFS implementations

[rkistner]

Currently, the InMemoryFileSystem uses a Uint8List buffer per file. When resizing the buffer, it creates a new one and copies the content over.

While copying the bytes is fairly fast for small buffers, doing that for every written page for a 10MB+ file size adds significant overhead. Basically, creating a database with size n takes O(n^2) time, which becomes very noticeable with large databases. This affects both IndexedDbFileSystem and InMemoryFileSystem, since IndexedDbFileSystem uses InMemoryFileSystem internally. In some of my tests, creating a 30MB database takes around 120s with either VFS.

This introduces a DynamicBuffer class that handles resizing the buffer. Instead of resizing on every write, it doubles the size every time new capacity is needed. This changes it to O(n) to create a database of size n. In my tests, the same database takes 5s to create with InMemoryFileSystem, and 10s with IndexedDbFileSystem.

Another approach would be to use the JS ArrayBuffer.resize together with maxByteLength, since ArrayBuffers are used internally for ByteBuffer with dart2js anyway. However, it may not be supported by all browsers, and would need a different approach for dart2wasm.

In my testing this appears to work well with both IndexedDbFileSystem and InMemoryFileSystem. It would still be good to review the IndexedDbFileSystem implementation specifically - I'm not sure whether the change could introduce some edge case, especially around previousContent.

[simolus3]

Thanks! I agree with the better management for handling growing data.

If possible, I prefer using an Uint8Buffer from package:typed_data for this though. That class is supposed to be doing the same thing, it's a core Dart package and one less thing to maintain here :)

[rkistner]

Great, I wasn't aware of that package - switched over now.

[simolus3]

Thanks! I've merged this and will upload a this weekend.