Restore runtime type checks for dynamic to class conversions

Note that this will generate exceptions when the value "undefined" is
assigned to a class (since undefined is actually a class instance). Fix
for that will be forthcoming.
Fix additional places using empty string in place of "null"
Remove "TODOs" that have been resolved
Remove parallel restriction from playc_tests Makefile
Add separate BuiltinType for the ActionScript "*" type
Add AsUntyped attribute to specify type "*" in C#

Using the AsUntyped attribute with a dynamic type instructs the
compiler to use the internal compiler type AsUntyped (representing the
"*" type) instead of the default internal type, Dynamic.