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

sky/engine/tonic/dart_state.h

Index: sky/engine/tonic/dart_state.h
diff --git a/sky/engine/tonic/dart_state.h b/sky/engine/tonic/dart_state.h
index 87431cbbd5b05d20dab6f5d58d32c875ae2375dd..9602b7e75a9dcf709d49cc1bbb417631f640c477 100644
--- a/sky/engine/tonic/dart_state.h
+++ b/sky/engine/tonic/dart_state.h
@@ -50,6 +50,7 @@ class DartState : public base::SupportsUserData {
   void set_isolate(Dart_Isolate isolate) {
     CHECK(!isolate_);
     isolate_ = isolate;
+    DidSetIsolate();
   }
 
   DartClassLibrary& class_library() { return *class_library_; }
@@ -57,6 +58,8 @@ class DartState : public base::SupportsUserData {
   DartExceptionFactory& exception_factory() { return *exception_factory_; }
   DartTimerHeap& timer_heap() { return *timer_heap_; }
 
+  virtual void DidSetIsolate() {}
+
  private:
   Dart_Isolate isolate_;
   OwnPtr<DartClassLibrary> class_library_;