onHandlesReady waits for planHandles (#6404)
onHandlesReady waits for planHandles (#6404)

Discovered during hackathon

Closes #6404

COPYBARA_INTEGRATE_REVIEW=https://github.com/polymerlabs/arcs/pull/6404 from cromwellian:wait-for-planhandles 4148bda4d144a1ce3116b2d9f3058789ab73a69b
PiperOrigin-RevId: 337988417

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs