speedup watchKey by preventing for in related deopt

another tiny microperf

keyName in obj if obj is not really enumerable or a complex object causes the entire function to deopt. As MANDATORY_SETTER is only a development thing, in production we don’t want it’s guard deopting us.

Someday this whole block should be stripped for production builds
Sorry to be so pedantic, but I think he meant switching `e` for `a`, i.e. `handleMandatorySetter`.