Use Object.create(null) when making maps.

Object.create(null) creates an object which doesn't inherit from Object. Thus:
'hasOwnProperty' in {} === true
but
'hasOwnProperty' in Object.create(null) === false

This makes it safer to use as a map, and with the 'in' operator.

Change-Id: I6c4947579d59f2b550a8bb2fe178de63bd554827
Merge "Use Object.create(null) when making maps."