TODO: syncnote
runtime reflection no longer uses atPhase
lock for runtimeMirror
synchronizes toolboxes

We don't guarantee thread-safety of the front-end, but everything else
looks good now.
fixup
TODO: syncnote
synchronizes the main culprit: makeScalaPackage
synchronizes top-level Class/ModuleSymbol materialization
the sync test
moves core synthetic classes materialization to init
wip
minor
yay!!
detailed explanation