code: function concat(a, b) {
    if (a.concat) { // Is `a` an Array?
      return a.concat(b);
    }
    if (a instanceof Blob) {
      return new Blob([a,b], {type: a.type});
    }
    if (a.set) { // Is `a` a typed array?
      var c = new a.constructor(a.length + b.length);
      c.set(a);
      c.set(b, a.length);
      return c;
    }
    throw new Error("Unknown data type");
  }
docs: /*
  Typed arrays and blobs don't have a concat method.
  This function helps StreamSource accumulate data to reach chunkSize.