Merges params from `source` into `target` (mutating `target`).

@param {!Object<string, !Array<string>>} target
@param {!Object<string, !Array<string>>} source
function mergeParams(target, source) {
    Object.keys(source).forEach(function (key) {
        if (!(key in target)) {
            target[key] = [];
        }
        target[key] = target[key].concat(source[key]);
    });
}