08d1c47ac3
This fixes the most common (and rather annoying) bug in augmented assignment desugaring with collection element receiver. Fix is somewhat hackish: introduce an intrinsic for MutableMap.set, thus bypassing discrepancies in 'get' and 'set' call generation. Fixing it properly requires design decisions for corner cases where ad hoc augmented assignment desugaring with collection element receiver "accidentally" works, producing identical objects and vararg arrays for arguments of 'get' and 'set'.