Add a LazyStream class.

test/lazy_stream_test.dart

+// Copyright (c) 2015, 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 "package:async/async.dart";
+import "package:test/test.dart";
+
+import "utils.dart";
+
+main() {
+  test("calls the callback when the stream is listened", () async {
+    var callbackCalled = false;
+    var stream = new LazyStream(expectAsync(() {
+      callbackCalled = true;
+      return new Stream.empty();
+    }));
+
+    await flushMicrotasks();
+    expect(callbackCalled, isFalse);
+
+    stream.listen(null);
+    expect(callbackCalled, isTrue);
+  });
+
+  test("forwards to a synchronously-provided stream", () async {
+    var controller = new StreamController<int>();
+    var stream = new LazyStream(expectAsync(() => controller.stream));
+
+    var events = [];
+    stream.listen(events.add);
+
+    controller.add(1);
+    await flushMicrotasks();
+    expect(events, equals([1]));
+
+    controller.add(2);
+    await flushMicrotasks();
+    expect(events, equals([1, 2]));
+
+    controller.add(3);
+    await flushMicrotasks();
+    expect(events, equals([1, 2, 3]));
+
+    controller.close();
+  });
+
+  test("forwards to an asynchronously-provided stream", () async {
+    var controller = new StreamController<int>();
+    var stream = new LazyStream(expectAsync(() async => controller.stream));
+
+    var events = [];
+    stream.listen(events.add);
+
+    controller.add(1);
+    await flushMicrotasks();
+    expect(events, equals([1]));
+
+    controller.add(2);
+    await flushMicrotasks();
+    expect(events, equals([1, 2]));
+
+    controller.add(3);
+    await flushMicrotasks();
+    expect(events, equals([1, 2, 3]));
+
+    controller.close();
+  });
+
+  test("a lazy stream can't be listened to multiple times", () {

[Lasse Reichstein Nielsen, 2015/12/03 11:44:20]

Check that stream.isBroadcast is false.

[nweiz, 2015/12/11 00:06:57]

Done.

+    var stream = new LazyStream(expectAsync(() => new Stream.empty()));
+    stream.listen(null);
+    expect(() => stream.listen(null), throwsStateError);
+    expect(() => stream.listen(null), throwsStateError);
+  });
+
+  test("a lazy stream can't be listened to from within its callback", () {
+    var stream;
+    stream = new LazyStream(expectAsync(() {
+      expect(() => stream.listen(null), throwsStateError);
+      return new Stream.empty();
+    }));
+    stream.listen(null);
+  });
+}