Add reactive-inspired stream transformers:

lib/src/transformers/throttle.dart

Index: lib/src/transformers/throttle.dart
diff --git a/lib/src/transformers/throttle.dart b/lib/src/transformers/throttle.dart
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+// Copyright (c) 2016, 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 "debounce.dart";
+
+/// Drops any event that happens shortly after another undropped event.
+///
+/// If an event happens within a certain [Duration] of the most recently
+/// output event, the new event is dropped.
+/// When the duration has passed after an event that wasn't dropped,
+/// the next event will be accepted.
+///
+/// This differs from [Debounce] which restarts the timer
+/// after dropped events too.
+class Throttle<S> implements StreamTransformer<S, S> {
+  final Duration _interval;
+  final Function _createStopwatch;
+
+  /// Drops events sooner than [interval] after emitted events.
+  ///
+  /// A [createStopwatch] function can be provided to introduce a non-standard
+  /// [Stopwatch] implementation, e.g., for testing.
+  /// It defaults to using [StopWatch].
+  Throttle(Duration interval, {Stopwatch createStopwatch()})
+      : _interval = interval,
+        _createStopwatch = createStopwatch;
+
+  Stream<S> bind(Stream<S> stream) async* {
+    var interval = Duration.ZERO;  // Avoid dropping the first event.
+    var stopwatch =
+        (_createStopwatch == null) ? new Stopwatch() : _createStopwatch();
+    stopwatch.start() ;
+    await for (var source in stream) {
+      if (stopwatch.elapsed >= interval) {
+        yield source;
+        stopwatch.reset();
+        interval = _interval;
+      }
+    }
+    stopwatch.stop();
+  }
+}