Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_gc_context.cc

+// Copyright 2015 The Chromium 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 "sky/engine/config.h"
+#include "sky/engine/tonic/dart_gc_context.h"
+
+namespace blink {
+
+DartGCContext::DartGCContext() : builder_(Dart_NewWeakReferenceSetBuilder()) {
+}
+
+DartGCContext::~DartGCContext() {
+}
+
+Dart_WeakReferenceSet DartGCContext::AddToSetForRoot(
+    const void* root,
+    Dart_WeakPersistentHandle handle) {
+  const auto& it = references_.add(root, nullptr);
+  if (!it.isNewEntry) {
+    Dart_AppendToWeakReferenceSet(it.storedValue->value, handle, handle);
+    return it.storedValue->value;
+  }
+  it.storedValue->value = Dart_NewWeakReferenceSet(builder_, handle, handle);
+  return it.storedValue->value;
+}
+
+}  // namespace blink