Make PhaseOutput emit a new AssetNode whenever it changes outputs.

Make PhaseOutput emit a new AssetNode whenever it changes outputs.

Previously, PhaseOutput produced a single AssetNode whose value it
would update whenever the forwarded output changed. This worked well
for a while, but once Phase started caring about the AssetNode's
origin, it became problematic. PhaseOutput would change the origin,
but Phase would have no way of knowing this, and the AssetNode would
end up sorted into the wrong place (treated as a forwarded node when
it shouldn't be, or vice versa).

This CL fixes that issue by removing the possibility of modifying an
AssetNode's origin. PhaseOutput now just creates a new AssetNode
instead. This has the additional benefit of cleaning up some
duplicated code in Phase.

R=rnystrom@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=28386

[Bob Nystrom, 2013-10-08 21:07:36]

One request then LGTM.

https://codereview.chromium.org/26413003/diff/1/pkg/barback/test/package_grap...
File pkg/barback/test/package_graph/group_test.dart (right):

https://codereview.chromium.org/26413003/diff/1/pkg/barback/test/package_grap...
pkg/barback/test/package_graph/group_test.dart:432: class
CheckContentAndRenameTransformer extends MockTransformer {
Move this to its own file in test/transformer.

[nweiz, 2013-10-08 22:20:12]

https://codereview.chromium.org/26413003/diff/1/pkg/barback/test/package_grap...
File pkg/barback/test/package_graph/group_test.dart (right):

https://codereview.chromium.org/26413003/diff/1/pkg/barback/test/package_grap...
pkg/barback/test/package_graph/group_test.dart:432: class
CheckContentAndRenameTransformer extends MockTransformer {
On 2013/10/08 21:07:36, Bob Nystrom wrote:
> Move this to its own file in test/transformer.

Done. I didn't do that originally because it seemed so specific to the test that
uses it here, but sure.

[nweiz, 2013-10-08 22:20:44]

Committed patchset #2 manually as r28386 (presubmit successful).