Refactor code by adding AssetProvider

[yuwen-yan]

This is the first PR to support #202

If this PR is acceptable, I'll add network_flare.dart under provider/ in next PR

[luigi-rosso]

I'm reviewing this now, it looks great. I just want to double-check how the new loading logic interacts with the cache (.load and .onLoaded). One thing that would be nice if this could be done on master instead of stable. We normally work out of master and then merge to dev and then finally to stable when we publish on pub.dev. We did that to mimic the Flutter repo, although we've heard from many that it's counter-intuitive.

I'm going to merge this into a new branch from master and push it up for us to work on (I'll add you as a collaborator). I'm also getting the examples working on it. Thanks so much for the contribution, excited to finally make this better.

[luigi-rosso]

I'm closing this as #214 landed which includes these changes into master.