In addition to the artifacts already consumed by build targets, I'm reasonably confident that the dependency tree can be implemented with a "Depends On" typeahead for build steps, and an implicit artifact created from previous build targets. This would allow the construction of advanced dependency trees between build steps, like so:

    a
   /|\
  / | \
 /  |  \
/   |   \
b   c   d
\   | 
 \  |
  \ |
   \|    
    e____
   /|\   \
  / | \   \
 /  |  \   \
/   |   \   \
f   g   h   i
\   |    \  | 
 \  |     \ |
  \ |      \|
   \|       j
    k

The build is considered successful only if all targets are currently in a successful state (or more accurately, we can re-run the steps in "failure steps", "success steps" when there are no more targets running to re-evaluate the build state).