We can't have two copies of |embedder.dart| or |shell.dart| because they take
ownership of some underlying Mojo handles. Instead of duplicating the code,
this CL makes the old locations just export all the symbols from the new
location. I've also done the same with fetch.dart to avoid code duplication.

Finally, I've removed image_cache.dart in the old location because the only
clients already live in the new world and ought to use the new location.

TBR=ianh@google.com

Review URL: https://codereview.chromium.org/1179923004.