Added "dartc_bot" target in dart.gyp

Added "dartc_bot" target in dart.gyp

Currently the dartc bots only use 'tools/build.py create_sdk'. But since dartc
analzes tests that use package imports this causes issues. Although we pass in
the package root (i.e. 'dart_analzer --package-root=out/Mode/packages/ ... '),
we don't create the package links on the bot (in the create_sdk step).

This CL creates a new 'dartc_bot' target which depends on the 'packages' target in
addition to 'create_sdk'. Another CL will change the buildbot to use the new
target.

Committed: https://code.google.com/p/dart/source/detail?r=18523

[ricow1, 2013-02-14 17:22:29]

LGTM

[ahe, 2013-02-14 19:03:41]

Actually, dartc should build everything. This is something we agreed upon a long
time ago because it otherwise finishes so quickly.