Make exclude list also exclude contexts

Make exclude list also exclude contexts

- Only create a context if the directory/.packages/pubspec.yaml is not excluded.
- Add tests.

Fixes: https://github.com/dart-lang/sdk/issues/23941

R=brianwilkerson@google.com, devoncarew@google.com, paulberry@google.com, pquitslund@google.com

Committed: https://github.com/dart-lang/sdk/commit/dfc35faaf940cc16c5d121e547d2c79c53cf1813

[Cutch, 2015-08-03 13:39:10]

johnmccutchan@google.com changed reviewers:
+ brianwilkerson@google.com, devoncarew@google.com, sethladd@google.com

[devoncarew, 2015-08-03 20:06:49]

devoncarew@google.com changed reviewers:
+ pquitslund@google.com

[devoncarew, 2015-08-03 20:06:49]

lgtm

Lgtm, but should also get a review from someone more familiar with this code.
Also adding pquitslund as I believe he's worked in this area.

[Brian Wilkerson, 2015-08-03 20:08:19]

brianwilkerson@google.com changed reviewers:
+ paulberry@google.com

[Brian Wilkerson, 2015-08-03 20:09:27]

I added Paul because he's making changes in this same area and I want to know
that they won't conflict with these changes. Other than that, LGTM.

[Paul Berry, 2015-08-03 20:49:05]

lgtm

[pquitslund, 2015-08-03 20:54:05]

LGTM!

[Cutch, 2015-08-03 20:54:25]

Committed patchset #2 (id:20001) manually as
dfc35faaf940cc16c5d121e547d2c79c53cf1813 (presubmit successful).