namedContext(name) {
    if (typeof name !== 'string') name = 'default';
    if (!this._validationContexts[name]) {
      this._validationContexts[name] = new ValidationContext(this, name);
    }
    return this._validationContexts[name];
  }
Creates and stores a new named (scoped) ValidationContext for a given name
and this schema and returns it.
@param name {string}
@return {ValidationContext}