Add `byteCollector` stream transformer and `collectBytes` function.

lib/src/byte_collector.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "dart:async";
+import "dart:typed_data";
+
+/// Collects an asynchronous sequence of byte lists into a single list of bytes.
+///
+/// If the [source] stream emits an error event,
+/// the collection fails and the returned future completes with the same error.
+///
+/// If any of the input data are not valid bytes, they will be truncated to
+/// an eight-bit unsigned value in the resulting list.
+Future<Uint8List> collectBytes(Stream<List<int>> source) {
+  var byteLists = List<List<int>>[];
+  var length = 0;
+  var completer = new Completer<Uint8List>.sync();
+  source.listen(
+      (bytes) {
+        byteLists.add(bytes);
+        length += bytes.length;
+      },
+      onError: completer.completeError,
+      onDone: () {
+        completer.complete(_collect(length, byteLists));
+      },
+      cancelOnError: true);
+  return completer.future;
+}
+
+// Join a lists of bytes with a known total length into a single [Uint8List].
+Uint8List _collect(int length, List<List<int>> byteLists) {
+  var result = new Uint8List(length);
+  int i = 0;
+  for (var byteList in byteLists) {
+    var end = i + byteList.length;
+    result.setRange(i, end, byteList);
+    i = end;
+  }
+  return result;
+}