Update charted to 0.4.8 and roll

packages/async/lib/src/lazy_stream.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 "delegate/stream.dart";
+import "stream_completer.dart";
+import "utils.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.
+  FutureOrCallback<Stream<T>> _callback;
+
+  /// Creates a single-subscription `Stream` that calls [callback] when it gets
+  /// a listener and forwards to the returned stream.
+  LazyStream(FutureOr<Stream<T>> 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<T> stream;
+    if (result is Future<Stream<T>>) {
+      stream = StreamCompleter.fromFuture(result.then((stream) {
+        return DelegatingStream.typed<T>(stream);
+      }));
+    } else {
+      stream = DelegatingStream.typed<T>(result as Stream);
+    }
+
+    return stream.listen(onData,
+        onError: onError, onDone: onDone, cancelOnError: cancelOnError);
+  }
+}