function BaseClient(backendClass, options) {
        /** Array of used integrations. */
        this._integrations = {};
        /** Number of calls being processed */
        this._numProcessing = 0;
        this._backend = new backendClass(options);
        this._options = options;
        if (options.dsn) {
            this._dsn = makeDsn(options.dsn);
        }
    }
Initializes this client instance.

@param backendClass A constructor function to create the backend.
@param options Options for the client.