The GWT compiler swamps a system when it builds, using multiple
threads to build each permutation in parallel. It also requires
a lot of RAM. Buck is not aware of the cost of the genrule and
currently schedules both to build at the same time.
Make ui_opt depend on ui_dbg so Buck is forced to serialize these
build steps, reducing the load on average systems while possibly
extending build time on very powerful (e.g. 16 core) systems.
Change-Id: Icc1856c9c4e7919c28dc533e792fa25aadcdf801
b547ca9d7b