Merge the tonic layer from skydart branch back to master

sky/engine/tonic/dart_isolate_scope.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_isolate_scope.h"
+
+namespace blink {
+
+DartIsolateScope::DartIsolateScope(Dart_Isolate isolate) {
+  isolate_ = isolate;
+  previous_ = Dart_CurrentIsolate();
+  if (previous_ == isolate_)
+    return;
+  if (previous_)
+    Dart_ExitIsolate();
+  Dart_EnterIsolate(isolate_);
+}
+
+DartIsolateScope::~DartIsolateScope() {
+  DCHECK(Dart_CurrentIsolate() == isolate_);
+  if (previous_ == isolate_)
+    return;
+  Dart_ExitIsolate();
+  if (previous_)
+    Dart_EnterIsolate(previous_);
+}
+
+}  // namespace blink