Add disabled tests for optional nullable arguments with non-null default values.
Inline dataLoc in the one place that can see it and the one place that can't.
Avoid a type error with a non-null default value for an optional nullable primitive argument.
Avoid a type error with a non-null default value for an optional nullable string argument.
auto merge of #1846 : Ms2ger/servo/non-null-default, r=jdm