Serialize barback assets more efficiently.

This adds an [InternalAsset] class to barback that supports serialization
operations, and specializes those operations for each asset type (for example,
FileAssets will just send the file path across isolates). This class isn't
exposed publicly, but pub imports it and uses it to handle asset serialization.

BUG=14430
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//55983005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@30126 260f80e4-7a28-3924-810f-c04153c831b5