code: static String getBuiltInBindingsKey(String key) {
      int start = startOfType(key);
      if (substringStartsWith(key, start, PROVIDER_PREFIX)) {
        return extractKey(key, start, key.substring(0, start), PROVIDER_PREFIX);
      } else if (substringStartsWith(key, start, MEMBERS_INJECTOR_PREFIX)) {
        return extractKey(key, start, "members/", MEMBERS_INJECTOR_PREFIX);
      } else {
        return null;
      }
    }
docs: Returns a key for the type provided by, or injected by this key. For
  example, if this is a key for a {@code Provider<Foo>}, this returns the
  key for {@code Foo}. This retains annotations and supports both Provider
  keys and MembersInjector keys.