Implement Mutex for Dart.

pkg/analyzer/lib/src/dart/analysis/mutex.dart

+// Copyright (c) 2017, 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';
+
+/// Mutual exclusion.
+///
+/// Usage:
+///
+///     var m = new Mutex();
+///
+///     await m.acquire();
+///     try {
+///       // critical section
+///     }
+///     finally {
+///       m.release();
+///     }

[Brian Wilkerson, 2017/08/25 16:56:44]

You might consider adding a utility method similar to:

Future<Null> guard(void criticalSection()) {
  await acquire();
  try {
    criticalSection();
  } finally {
    release();
  }
}

[scheglov, 2017/08/25 17:18:28]

Done.

+class Mutex {
+  Completer<Null> _lock;
+
+  /// Acquire a lock.
+  ///
+  /// Returns a [Future] that will be completed when the lock has been acquired.
+  Future<Null> acquire() async {
+    while (_lock != null) {
+      await _lock.future;
+    }
+    _lock = new Completer<Null>();
+  }
+
+  /// Release a lock.
+  ///
+  /// Release a lock that has been acquired.
+  void release() {
+    if (_lock == null) {
+      throw new StateError('No lock to release.');
+    }
+    _lock.complete();
+    _lock = null;
+  }
+}