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

sky/engine/core/script/dom_dart_state.cc

Index: sky/engine/core/script/dom_dart_state.cc
diff --git a/sky/engine/core/script/dom_dart_state.cc b/sky/engine/core/script/dom_dart_state.cc
index 8f5784b58259a3bb6bd310deffb4bd253daf3a8e..2e2d8ecc7d056c24fbe1b47f873688e3b9808c13 100644
--- a/sky/engine/core/script/dom_dart_state.cc
+++ b/sky/engine/core/script/dom_dart_state.cc
@@ -11,12 +11,20 @@
 namespace blink {
 
 DOMDartState::DOMDartState(Document* document)
-    : document_(document), loader_(adoptPtr(new DartLoader(this))) {
+    : document_(document),
+      loader_(adoptPtr(new DartLoader(this))) {
 }
 
 DOMDartState::~DOMDartState() {
 }
 
+void DOMDartState::DidSetIsolate() {
+  Scope dart_scope(this);
+  x_handle_.Set(this, Dart_NewStringFromCString("x"));
+  y_handle_.Set(this, Dart_NewStringFromCString("y"));
+  value_handle_.Set(this, Dart_NewStringFromCString("_value"));
+}
+
 DOMDartState* DOMDartState::Current() {
   return static_cast<DOMDartState*>(DartState::Current());
 }
@@ -35,4 +43,4 @@ LocalDOMWindow* DOMDartState::CurrentWindow() {
   return Current()->document_->domWindow();
 }
 
-}
+}  // namespace blink