Specifically, the following changes:
1) A union that includes a dictionary and is the last non-optional
argument must actually be marked optional, just like a dictionary
argument.
2) Disallow a union from containing both a nullable type and a dictionary.
3) Now all non-Date and non-RegExp objects can be used as dictionaries,
including from overload resolution and union conversion.
We don't support dictionaries inside unions yet, or unions as
distinguishing args, so the spec changes to do with converting null to
dictionaries inside a union or picking the union overload if there is
a union containing a dictionary and null is the distinguishing arg
value are not relevant to us so far.
