The goal of this change is to make it easy to synchronously propagate the dirty
state of an asset node to all of the nodes and transforms that depend on it,
while still asynchronously processing those transforms.
In addition, this refactor ends up addressing two TODOs and one bug:
* If a node has been compiled previously and an unrelated part of its cascade is
dirtied, it will now be returned immediately rather than waiting for the
entire cascade to finish processing.
* If a node is consumed as the input of a transform, it will no longer be
returned by [getAssetById].
* If the contents of a transform's primary input changes such that it's no
longer a valid primary input for that transform, the transform will be
removed.
R=rnystrom@google.com
Review URL: https://codereview.chromium.org//21275003
git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@25693 260f80e4-7a28-3924-810f-c04153c831b5
kevmoo@j832.com