Fix an edge case in the dependency computer.

Fix an edge case in the dependency computer.

When a package both transformed itself and had a dev transformer (that
is, a transformer that only runs on non-public files), *and* that
package's self-transformed code was imported by another transformer, the
dependency computer could return the dev transformer in its results.

Closes #1290
Closes #1291

R=kevmoo@google.com

Committed: https://github.com/dart-lang/pub/commit/052756a1ed0832dea23743ec22c2165df45ba3c7

[nweiz, 2015-07-07 00:06:41]

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

[kevmoo, 2015-07-07 00:11:32]

lgtm

[nweiz, 2015-07-07 00:12:34]

Committed patchset #1 (id:1) manually as
052756a1ed0832dea23743ec22c2165df45ba3c7 (presubmit successful).