code: @GwtIncompatible("Unnecessary")
    private Map<String, String> constructRootRelativePathsMap() {
      Map<String, String> rootRelativePathsMap = new LinkedHashMap<>();
      for (String mapString : config.manifestMaps) {
        int colonIndex = mapString.indexOf(':');
        checkState(colonIndex > 0);
        String execPath = mapString.substring(0, colonIndex);
        String rootRelativePath = mapString.substring(colonIndex + 1);
        checkState(rootRelativePath.indexOf(':') == -1);
        rootRelativePathsMap.put(execPath, rootRelativePath);
      }
      return rootRelativePathsMap;
    }
docs: Construct and return the input root path map. The key is the exec path of each input file, and
  the value is the corresponding root relative path.