Fix error detection for a non-existent global script.

Fix error detection for a non-existent global script.

When we tried to run a global script that didn't exist, we would try to
load an AssetEnvironment, which would try to compute the transformers
for it. Because the script doesn't exist, the computation would
conservatively assume that all transformers had to run in case one of
them generated it.

To find all the scripts, it iterated through the package's dependencies.
Since it's notionally the entrypoint, it tried to include its dev
dependencies, but this broke because dev dependencies aren't installed
for globally-activated packages.

R=rnystrom@google.com

Committed: https://github.com/dart-lang/pub/commit/a1139b80c0230a429a468f9f411556a3fc183bba

[Bob Nystrom, 2015-10-28 16:01:11]

lgtm

[nweiz, 2015-10-28 21:24:16]

Committed patchset #2 (id:20001) manually as
a1139b80c0230a429a468f9f411556a3fc183bba (presubmit successful).