There are several parts here:
1) Enforce the requirement that dictionary arguments not followed by a required argument are optional.
2) Make dictionaries no longer be distinguishable from nullable types.
3) Disallow dictionaries or unions containing dictionaries inside a nullable type.
4) Force optional dictionaries to have a default value of null so that codegen doesn't have to worry about dealing with
optional arguments that have no default value in the IDL but need to be treated as if they were null.
There are several parts here:
1) Enforce the requirement that dictionary arguments not followed by a required argument are optional.
2) Make dictionaries no longer be distinguishable from nullable types.
3) Disallow dictionaries or unions containing dictionaries inside a nullable type.
4) Force optional dictionaries to have a default value of null so that codegen doesn't have to worry about dealing with
optional arguments that have no default value in the IDL but need to be treated as if they were null.
Another implementation option would be to put all the dictionaries in a single
file and have a static global set of ids which works across all dictionaries
and is initialized once at startup or so. That would also handle cross-file
dictionary inheritance better.
One problem that remains is the fake descriptor business. At the moment this
does not allow interface types inside dictionaries. We could probably work
around this by either refactoring code to make it possible to get the declType
independently of the actual conversion template (whether because it lives in a
separate function or because the conversion template generator knows to just
return an empty string when the fake descriptor provirder is passed) or by
figuring out a way to pass an actual descriptor provider to dictionary codegen.
