We were validating that the dependencies and dev_dependencies had no overlap
whenever we accessed [Pubspec.dependencies], but this meant that we had to parse
the dev_dependencies even if they were going unused. If that parse failed, it
could stop an entire "pub get" from working. Now we only do this validation when
[Pubspec.devDependencies] is accessed.

This also fixes a bug in where, if a pubspec from outside the filesystem (e.g. a
hosted dependency) has a relative path dependency, [PathSource.parseDescription]
would crash. Although this shouldn't happen since we don't parse the dev
dependencies anymore, [PathSource] will now handle it more gracefully if it
does.

Finally, this also works around an async_await bug that was causing stack traces
to be truncated.

R=rnystrom@google.com
BUG=22194

Review URL: https://codereview.chromium.org//887483002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@43267 260f80e4-7a28-3924-810f-c04153c831b5