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

test/byte_collection_test.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";
+
+import "package:test/test.dart";
+import "package:async/async.dart" show byteCollector, collectBytes, Result;
+
+void main() {
+  group("collectBytes", () {
+    test("simple list and overflow", () {
+      var result = collectBytes(new Stream.fromIterable([
+        [0],
+        [1],
+        [2],
+        [256]
+      ]));
+      expect(result, completion([0, 1, 2, 0]));
+    });
+
+    test("no events", () {
+      var result = collectBytes(new Stream.fromIterable([]));
+      expect(result, completion([]));
+    });
+
+    test("empty events", () {
+      var result = collectBytes(new Stream.fromIterable([[], []]));
+      expect(result, completion([]));
+    });
+
+    test("error event", () {
+      var result = collectBytes(new Stream.fromIterable(
+          new Iterable.generate(3, (n) => n == 2 ? throw "badness" : [n])));
+      expect(result, throwsA("badness"));
+    });
+  });
+}