BCD has some data for Web IDL typedefs, namely ConstrainBoolean, ConstrainDOMString, ConstrainDouble, ConstrainULong and DOMHighResTimestamp.

Typedefs in Web IDL are aliases of other types which are invisible to web developers. DOMHighResTimestamp when converted to JavaScript is a Number, but one that has a certain interpretation.

Because typedefs are aliases, they have no compat story of their own separately from the APIs that use the typedefs. In this way they are similar to dictionaries, but even more trivial.

This came up in #9892.