Package missingtypes contains types that can't be found.
As webapi packages is having WebIDL files that are extracted from different specification files, they may sometimes be missing a types. This package just define that type to get the binder program that is generating webapi code to be happy.
DateFromJS is casting a js.Wrapper into Date.
DictionaryFromJS is casting a js.Wrapper into Dictionary.
WritableStreamFromJS is casting a js.Wrapper into WritableStream.