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

sky/engine/core/painting/Rect.cpp

 // 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/core/painting/Rect.h"
 
+#include "sky/engine/core/script/dom_dart_state.h"
 #include "sky/engine/tonic/dart_error.h"
-#include "sky/engine/tonic/dart_state.h"
+#include "sky/engine/tonic/dart_value.h"
 #include "base/logging.h"
 
 namespace blink {
 
 // Convert dart_rect._value[0...3] ==> SkRect.
 Rect DartConverter<Rect, void>::FromArgumentsWithNullCheck(
     Dart_NativeArguments args,
     int index,
     Dart_Handle& exception) {
   Rect result;
   result.is_null = true;
 
   Dart_Handle dart_rect = Dart_GetNativeArgument(args, index);
   DCHECK(!LogIfError(dart_rect));
 
   Dart_Handle value = Dart_GetField(dart_rect,
-                                    Dart_NewStringFromCString("_value"));
+                                    DOMDartState::Current()->value_handle());
   if (Dart_IsNull(value))
     return result;
 
   Dart_TypedData_Type type;
   float* data = nullptr;
   intptr_t num_elements = 0;
   Dart_TypedDataAcquireData(
       value, &type, reinterpret_cast<void**>(&data), &num_elements);
   DCHECK(!LogIfError(value));
   ASSERT(type == Dart_TypedData_kFloat32 && num_elements == 4);
 
   SkScalar* dest[] = {
     &result.sk_rect.fLeft,
     &result.sk_rect.fTop,
     &result.sk_rect.fRight,
     &result.sk_rect.fBottom
   };
   for (intptr_t i = 0; i < 4; ++i)
     *dest[i] = data[i];
 
   Dart_TypedDataReleaseData(value);
 
   result.is_null = false;
   return result;
 }
 
 } // namespace blink