Add a LazyStream class.

lib/src/lazy_stream.dart

Index: lib/src/lazy_stream.dart
diff --git a/lib/src/lazy_stream.dart b/lib/src/lazy_stream.dart
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index 0000000000000000000000000000000000000000..1878d4cfb30ec3547f9ed36a144ccc00ed23e305
--- /dev/null
+++ b/lib/src/lazy_stream.dart
@@ -0,0 +1,51 @@
+// 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.
+
+library async.lazy_stream;
+
+import "dart:async";
+
+import "stream_completer.dart";
+
+/// A [Stream] wrapper that forwards to another [Stream] that's initialized
+/// lazily.
+///
+/// This class allows a concrete `Stream` to be created only once it has a
+/// listener. It's useful to wrapping APIs that do expensive computation to
+/// produce a `Stream`.
+class LazyStream<T> extends Stream<T> {
+  /// The callback that's called to create the inner stream.
+  ZoneCallback _callback;
+
+  /// Creates a single-subscription `Stream` that calls [callback] when it gets
+  /// a listener and forwards to the returned stream.
+  ///
+  /// The [callback] may return a `Stream` or a `Future<Stream>`.
+  LazyStream(callback()) : _callback = callback {
+    // Explicitly check for null because we null out [_callback] internally.
+    if (_callback == null) throw new ArgumentError.notNull('callback');
+  }
+
+  StreamSubscription<T> listen(void onData(T event),
+                               {Function onError,
+                                void onDone(),
+                                bool cancelOnError}) {
+    if (_callback == null) {
+      throw new StateError("Stream has already been listened to.");
+    }
+
+    // Null out the callback before we invoke it to ensure that even while
+    // running it, this can't be called twice.
+    var callback = _callback;
+    _callback = null;
+    var result = callback();
+
+    Stream stream = result is Future
+        ? StreamCompleter.fromFuture(result)
+        : result;
+
+    return stream.listen(onData,
+        onError: onError, onDone: onDone, cancelOnError: cancelOnError);
+  }
+}