Add DartHandleCache and use it to cache strings used when converting Rect and

sky/engine/core/script/dom_dart_state.h

 // 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.
 
 #ifndef SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_
 #define SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_
 
 #include "dart/runtime/include/dart_api.h"
 #include "sky/engine/tonic/dart_state.h"
 #include "sky/engine/core/dom/Document.h"
 #include "sky/engine/wtf/RefPtr.h"
 
 namespace blink {
 class DartLoader;
 class LocalFrame;
 class LocalDOMWindow;
 
 class DOMDartState : public DartState {
  public:
   explicit DOMDartState(Document* document);
   ~DOMDartState() override;
 
+  virtual void DidSetIsolate();
+
   static DOMDartState* Current();
 
   static Document* CurrentDocument();
   static LocalFrame* CurrentFrame();
   static LocalDOMWindow* CurrentWindow();
 
   Document* document() const { return document_.get(); }
   DartLoader& loader() const { return *loader_; }
 
+  // Cached handles to strings used in Dart/C++ conversions.
+  Dart_Handle x_handle() { return x_handle_.value(); }
+  Dart_Handle y_handle() { return y_handle_.value(); }
+  Dart_Handle value_handle() { return value_handle_.value(); }
+
  private:
   RefPtr<Document> document_;
   OwnPtr<DartLoader> loader_;
+
+  DartPersistentValue x_handle_;
+  DartPersistentValue y_handle_;
+  DartPersistentValue value_handle_;
 };
 
-}
+}  // namespace blink
 
 #endif // SKY_ENGINE_CORE_SCRIPT_DOM_DART_STATE_H_