Add first version of runAsync.

sdk/lib/async/event_loop.dart

+// Copyright (c) 2013, 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.
+
+part of dart.async;
+
+typedef void _AsyncCallback();
+
+bool _callbacksAreEnqueued = false;
+List<_AsyncCallback> _asyncCallbacks = <_AsyncCallback>[];
+
+void _asyncRunCallback() {
+  // As long as we are iterating over the registered callbacks we don't
+  // unset the [_callbacksAreEnqueued] boolean.
+  while (!_asyncCallbacks.isEmpty) {
+    List callbacks = _asyncCallbacks;
+    // The callbacks we execute can register new callbacks. This means that
+    // the for-loop below could grow the list if we don't replace it here.
+    _asyncCallbacks = <_AsyncCallback>[];
+    for (int i = 0; i < callbacks.length; i++) {
+      Function callback = callbacks[i];
+      callbacks[i] = null;
+      try {
+        callback();
+      } catch (e) {
+        i++;  // Skip current callback.
+        List remainingCallbacks = callbacks.getRange(i, callbacks.length - i);
+        List newCallbacks = _asyncCallbacks;
+        _asyncCallbacks = <_AsyncCallback>[];
+        _asyncCallbacks.addAll(remainingCallbacks);
+        _asyncCallbacks.addAll(newCallbacks);
+        _AsyncRun._enqueueImmediate(_asyncRunCallback);
+        throw;
+      }
+    }
+  }
+  // Any new callback must register a callback function now.
+  _callbacksAreEnqueued = false;
+}
+
+/**
+ * Runs the given [callback] asynchronously.
+ *
+ * Callbacks registered through this function are always executed in order and
+ * are guaranteed to run before other asynchronous events (like [Timer] events,
+ * or DOM events).
+ *
+ * Warning: it is possible to starve the DOM by registering asynchronous
+ * callbacks through this method. For example the following program will
+ * run the callbacks without ever giving the Timer callback a chance to execute:
+ *
+ *     Timer.run(() { print("executed"); });  // Will never be executed;
+ *     foo() {
+ *       asyncRun(foo);  // Schedules [foo] in front of other events.
+ *     }
+ *     main() {
+ *       foo();
+ *     }
+ */
+void runAsync(void callback()) {
+  // Optimizing a group of Timer.run callbacks to be executed in the
+  // same Timer callback.
+  _asyncCallbacks.add(callback);
+  if (!_callbacksAreEnqueued) {
+    _AsyncRun._enqueueImmediate(_asyncRunCallback);
+    _callbacksAreEnqueued = true;
+  }
+}
+
+class _AsyncRun {
+  /** Enqueues the given callback before any other event in the event-loop. */
+  external static void _enqueueImmediate(void callback());
+}