Add a CountDownLatch to fml with tests.

ci/licenses_golden/licenses_flutter

@@ -495,6 +495,9 @@ FILE: ../../../flutter/fml/platform/win/errors_win.h
 FILE: ../../../flutter/fml/platform/win/file_win.cc
 FILE: ../../../flutter/fml/platform/win/native_library_win.cc
 FILE: ../../../flutter/fml/platform/win/wstring_conversion.h
+FILE: ../../../flutter/fml/synchronization/count_down_latch.cc
+FILE: ../../../flutter/fml/synchronization/count_down_latch.h
+FILE: ../../../flutter/fml/synchronization/count_down_latch_unittests.cc
 FILE: ../../../flutter/fml/unique_fd.cc
 FILE: ../../../flutter/fml/unique_fd.h
 FILE: ../../../flutter/fml/unique_object.h

fml/BUILD.gn

@@ -46,6 +46,8 @@ source_set("fml") {
     "paths.h",
     "string_view.cc",
     "string_view.h",
+    "synchronization/count_down_latch.cc",
+    "synchronization/count_down_latch.h",
     "synchronization/thread_annotations.h",
     "synchronization/thread_checker.h",
     "synchronization/waitable_event.cc",
@@ -176,6 +178,7 @@ executable("fml_unittests") {
     "message_unittests.cc",
     "paths_unittests.cc",
     "string_view_unittest.cc",
+    "synchronization/count_down_latch_unittests.cc",
     "synchronization/thread_annotations_unittest.cc",
     "synchronization/thread_checker_unittest.cc",
     "synchronization/waitable_event_unittest.cc",

fml/synchronization/count_down_latch.cc

@@ -0,0 +1,29 @@
+// Copyright 2018 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/fml/synchronization/count_down_latch.h"
+
+#include "flutter/fml/logging.h"
+
+namespace fml {
+
+CountDownLatch::CountDownLatch(size_t count) : count_(count) {
+  if (count_ == 0) {
+    waitable_event_.Signal();
+  }
+}
+
+CountDownLatch::~CountDownLatch() = default;
+
+void CountDownLatch::Wait() {
+  waitable_event_.Wait();
+}
+
+void CountDownLatch::CountDown() {
+  if (--count_ == 0) {
+    waitable_event_.Signal();
+  }
+}
+
+}  // namespace fml

fml/synchronization/count_down_latch.h

@@ -0,0 +1,29 @@
+// Copyright 2018 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <atomic>
+
+#include "flutter/fml/macros.h"
+#include "flutter/fml/synchronization/waitable_event.h"
+
+namespace fml {
+
+class CountDownLatch {
+ public:
+  CountDownLatch(size_t count);
+
+  ~CountDownLatch();
+
+  void Wait();
+
+  void CountDown();
+
+ private:
+  std::atomic_size_t count_;
+  ManualResetWaitableEvent waitable_event_;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(CountDownLatch);
+};
+
+}  // namespace fml

fml/synchronization/count_down_latch_unittests.cc

@@ -0,0 +1,39 @@
+// Copyright 2018 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <chrono>
+#include <thread>
+
+#include "flutter/fml/synchronization/count_down_latch.h"
+#include "flutter/fml/thread.h"
+#include "flutter/testing/testing.h"
+
+namespace fml {
+
+TEST(CountDownLatchTest, CanWaitOnZero) {
+  CountDownLatch latch(0);
+  latch.Wait();
+}
+
+TEST(CountDownLatchTest, CanWait) {
+  fml::Thread thread("test_thread");
+  const size_t count = 100;
+  size_t current_count = 0;
+  CountDownLatch latch(count);
+  auto decrement_latch_on_thread = [runner = thread.GetTaskRunner(), &latch,
+                                    &current_count]() {
+    runner->PostTask([&latch, &current_count]() {
+      std::this_thread::sleep_for(std::chrono::microseconds(100));
+      current_count++;
+      latch.CountDown();
+    });
+  };
+  for (size_t i = 0; i < count; ++i) {
+    decrement_latch_on_thread();
+  }
+  latch.Wait();
+  ASSERT_EQ(current_count, count);
+}
+
+}  // namespace fml