pkg/pool: remove usage of Chain.track

remove usage of Chain.track

The min SDK is already 1.9, so we're not leaving anyone behind

Also removed an unused variable in pool_test

R=nweiz@google.com

Committed: https://github.com/dart-lang/pool/commit/1f2f018a7b3c741deb7fd37111dab1c89011b282

[kevmoo, 2015-07-01 00:27:28]

kevmoo@google.com changed reviewers:
+ nweiz@google.com

[nweiz, 2015-07-01 00:44:35]

lgtm

https://codereview.chromium.org/1215053004/diff/1/lib/pool.dart
File lib/pool.dart (right):

https://codereview.chromium.org/1215053004/diff/1/lib/pool.dart#newcode90
lib/pool.dart:90: return request().then((resource) => new
Future.sync(callback).whenComplete(resource.release));
Long line.

https://codereview.chromium.org/1215053004/diff/1/pubspec.yaml
File pubspec.yaml (right):

https://codereview.chromium.org/1215053004/diff/1/pubspec.yaml#newcode2
pubspec.yaml:2: version: 1.2.0-dev
This should be 1.1.1 or 1.1.1-dev, since it doesn't add any user-visible
behavior.

[kevmoo, 2015-07-01 00:46:31]

https://codereview.chromium.org/1215053004/diff/1/pubspec.yaml
File pubspec.yaml (right):

https://codereview.chromium.org/1215053004/diff/1/pubspec.yaml#newcode2
pubspec.yaml:2: version: 1.2.0-dev
On 2015/07/01 00:44:35, nweiz wrote:
> This should be 1.1.1 or 1.1.1-dev, since it doesn't add any user-visible
> behavior.

Done - leaving the release until there is something helpful...

[kevmoo, 2015-07-01 00:46:40]

Committed patchset #2 (id:20001) manually as
1f2f018a7b3c741deb7fd37111dab1c89011b282 (presubmit successful).