async load (schema, fetchUrl, location) {
    this.schema = schema
    await this._asyncloadExternalRefs(schema, fetchUrl, this._getFileBase(location), true)
    return this.expandRefs(schema)
  }
Fully loads and expands JSON schema for a provided schema object and URL.

The callback receives a expanded JSON Schema object with references
replaced with loaded schemas.

@param {object} schema - A JSON Schema.
@param {string} fetchUrl - Base path from which to store the definitions.
  Typically the URI of the schema.
@param {*} location - The base URL from which to load relative paths.
@returns {object} A JSON Schema with references expanded.