code: public ClassSymbol enterClass(ModuleSymbol msym, Name flatname) {
          Assert.checkNonNull(msym);
          PackageSymbol ps = lookupPackage(msym, Convert.packagePart(flatname));
          Assert.checkNonNull(ps);
          Assert.checkNonNull(ps.modle);
          ClassSymbol c = getClass(ps.modle, flatname);
          if (c == null) {
              c = defineClass(Convert.shortName(flatname), ps);
              doEnterClass(ps.modle, c);
              return c;
          } else
              return c;
      }
docs: Create a new member or toplevel class symbol with given flat name
  and enter in `classes' unless already there.