Avoid O(n^2) behavior in Barback.

Avoid O(n^2) behavior in Barback.

This changes [Phase.isDirty] and [Phase.onDone] to consider a phase
dirty if any previous phase is also dirty. Not only is this more
efficient, it's more accurate, since now [Phase.isDirty] is true
exactly when it's possible that a phase will emit new outputs.

R=rnystrom@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=33447

[Bob Nystrom, 2014-03-07 17:05:58]

Couple of nits, but LGTM. Much cleaner now.

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.dart
File pkg/barback/lib/src/phase.dart (right):

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:96: /// Whether [this] is dirty and still has
more processing to do.
Add a bit here explaining that a phase is if it's doing work or its previous
phase is dirty.

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:110: StreamSubscription _previousSubscription;
_previousOnDoneSubscription?

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:146: this.onDone.listen((_) {
Remove "this.".

[nweiz, 2014-03-07 19:39:58]

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.dart
File pkg/barback/lib/src/phase.dart (right):

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:96: /// Whether [this] is dirty and still has
more processing to do.
On 2014/03/07 17:05:58, Bob Nystrom wrote:
> Add a bit here explaining that a phase is if it's doing work or its previous
> phase is dirty.

Done.

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:110: StreamSubscription _previousSubscription;
On 2014/03/07 17:05:58, Bob Nystrom wrote:
> _previousOnDoneSubscription?

Done.

https://codereview.chromium.org/189623006/diff/1/pkg/barback/lib/src/phase.da...
pkg/barback/lib/src/phase.dart:146: this.onDone.listen((_) {
On 2014/03/07 17:05:58, Bob Nystrom wrote:
> Remove "this.".

Done.

[nweiz, 2014-03-07 20:10:33]

Committed patchset #2 manually as r33447 (presubmit successful).