* Move asset:// URL handling to DefaultFileSource.
* AssetFileSource implements FileSource interface and follows familiar implementation patterns.
* Move default implementation to platform/default, zip implementation to platform/android.
* Don't bother with modified / expires / etag -- assets are not cached so it doesn't matter.
* Don't bother with interleaving individual IO calls on the implementation thread. That adds a lot of complexity for very little benefit.
In the cache response case, we don't want the failedRequests logic; cached errors, should they occur, shouldn't affect the scheduling behavior for real requests.
We already had a condition that checks if a response is immediately available for an incoming request. Call the callback there, rather than in addObserver.
In every case where setResponse is called, notify was called soon after. In one case immediately, in another, after intervening functions which are safe to reorder after notifications.
The null-check of response is safe to eliminate because response must not be null inside setResponse.
There is no need for the timer to be created only under certain conditions. Stopping a non-started timer is a no-op. Therefore it can simply be a direct member.
While here, rename the variable. It's a timer, not a request.
