Don't load transformers that aren't going to be used for an executable.

A package's executables may not use all of that package's transformers. This is
especially true for browser frameworks where there's a clear divide between
client-side and server-side code. This uses pub's existing dependency-sniffing
infrastructure to figure out which transformers can be omitted.

This takes "pub run polymer:new_element --help" from 1.52s to 0.96s.

R=rnystrom@google.com
BUG=20859

Review URL: https://codereview.chromium.org//599993004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40659 260f80e4-7a28-3924-810f-c04153c831b5