code: VariableMap getPropertyMap() {
      ImmutableMap.Builder<String, String> map = ImmutableMap.builder();
      for (Property p : propertyMap.values()) {
        if (p.newName != null) {
          map.put(p.oldName, p.newName);
        }
      }
      return new VariableMap(map.build());
    }
docs: Gets the property renaming map (the "answer key").
  
  @return A mapping from original names to new names