static add(key, dep) {
    if (!shaka.dependencies.Allowed[key]) {
      throw new Error(`${key} is not supported`);
    }
    if (key == shaka.dependencies.Allowed.muxjs) {
      shaka.Deprecate.deprecateFeature(5,
          'mux.js',
          'mux.js is no longer used in Shaka Player.');
      return;
    }
    shaka.dependencies.dependencies_.set(key, () => dep);
  }
Registers a new dependency.

@param {shaka.dependencies.Allowed} key which is used for retrieving a
  dependency
@param {?} dep a dependency
@export