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

sky/engine/core/painting/Rect.cpp

Index: sky/engine/core/painting/Rect.cpp
diff --git a/sky/engine/core/painting/Rect.cpp b/sky/engine/core/painting/Rect.cpp
index 4a0c643cc8bd09ed69f8c0e8da2e3fda36bd3ad7..625d45614e508d9bbbf12cf9f18087cc497fa126 100644
--- a/sky/engine/core/painting/Rect.cpp
+++ b/sky/engine/core/painting/Rect.cpp
@@ -5,8 +5,9 @@
 #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 {
@@ -23,7 +24,7 @@ Rect DartConverter<Rect, void>::FromArgumentsWithNullCheck(
   DCHECK(!LogIfError(dart_rect));
 
   Dart_Handle value = Dart_GetField(dart_rect,
-                                    Dart_NewStringFromCString("_value"));
+                                    DOMDartState::Current()->value_handle());
   if (Dart_IsNull(value))
     return result;