Impeller's HostBuffer is now a simple DeviceBuffer bump and no longer has any special lazy upload functionality.

Among other things, rewriting the HostBuffer as a Dart utility would simplify away the Buffer mixin dispatch that happens for all RenderPass calls that take a BufferView.
Also, Impeller's HostBuffer is tuned to Impeller's 2D renderer. Flutter GPU's bump allocator should allow for customization of the block size.