Make constants that have a value in the IDL non-external

[srujzs]

A number of APIs have constant values e.g. XMLHttpRequest. We currently emit them as external values, but we can emit them as Dart members since the values are known in the IDL e.g. XMLHttpRequest. The only downside here is that we're ignoring what the browser implements, but in practice, I don't expect that to be different from the IDL values. The benefit of doing this is to be able to use these values in a switch-case.

[kevmoo]

Didn't we discuss this somewhere else, too?

[kevmoo]

I agree!

[srujzs]

Didn't we discuss this somewhere else, too?

Yup! You brought this up somewhere in an external code review but searching my mail led me to no results.