message: fallback to mixins when merging partial interfaces
diff: @@ -70,7 +70,8 @@ function emitDom() {
  }
  for (const w of widlStandardTypes) {
  for (const partial of w.partialInterfaces) {
  - const base = webidl.interfaces!.interface[partial.name];
  + // Fallback to mixins before every spec migrates to `partial interface mixin`.
  + const base = webidl.interfaces!.interface[partial.name] || webidl.mixins!.mixin[partial.name];
  if (base) {
  resolveExposure(partial, base.exposed!);
  merge(base.constants, partial.constants, true);
  