`reflectClass` on record type crashes VM

[lrhn]

Example:

import "dart:mirrors";
typedef Rec = (int,);
void main() {
  reflectClass(Rec);
}

I know dart:mirrors are not really supported, but crashing with a segmentation fault seems a little harsher than necessary. Throwing UnuspportedError for unrecognized types seems safer than a native assertion failure.