Reorganize barback's source files.

Reorganize barback's source files.

This creates four directories beneath barback/lib/src:

* asset/ contains the Asset and AssetNode classes, as well as various
  classes for dealing with them.

* graph/ contains the classes that make up the barback asset graph, as
  well as utility classes that are specific to these (such as
  NodeStreams).

* transformer/ contains Transformer and similar classes (such as
  TransformerGroup), as well as the classes (such as Transform) that
  are passed to Transformer methods.

* utils/ contains utility classes that aren't barback-specific (such
  as CancelableFuture).

R=rnystrom@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35777

[Bob Nystrom, 2014-05-02 16:46:02]

lgtm

[nweiz, 2014-05-02 19:13:37]

PTAL

Since pub used barback/lib/src/internal_asset.dart, I added a stub that exports
the new location so pub can support the full range of barback versions.

[Bob Nystrom, 2014-05-05 20:37:22]

lgtm

[nweiz, 2014-05-05 22:59:38]

Committed patchset #2 manually as r35777 (presubmit successful).