code: private void computeDependence(final Definition def, Node rValue) {
      NodeTraversal.traverse(
          compiler,
          rValue,
          new AbstractCfgNodeTraversalCallback() {
            @Override
            public void visit(NodeTraversal t, Node n, Node parent) {
              if (n.isName()) {
                Var dep = allVarsInFn.get(n.getString());
                if (dep == null) {
                  def.unknownDependencies = true;
                } else {
                  def.depends.add(dep);
                }
              }
            }
          });
    }
docs: Computes all the local variables that rValue reads from and store that
  in the def's depends set.