Add external byte array API and finalize external strings and byte arrays.

runtime/vm/dart_api_state.h

Index: runtime/vm/dart_api_state.h
diff --git a/runtime/vm/dart_api_state.h b/runtime/vm/dart_api_state.h
index 9badcc4fb9447228d1ad0ca6c148d081686b6f06..942473ea407f1fc499305cd64205e53cbf496d5d 100644
--- a/runtime/vm/dart_api_state.h
+++ b/runtime/vm/dart_api_state.h
@@ -127,11 +127,15 @@ class WeakPersistentHandle {
   Dart_PeerFinalizer callback() const { return callback_; }
   void set_callback(Dart_PeerFinalizer callback) { callback_ = callback; }
 
-  void Finalize() {
-    if (callback_ != NULL) {
-      (*callback_)(peer_);
+  static void Finalize(WeakPersistentHandle* handle) {
+    if (handle->callback() != NULL) {
+      Dart_PeerFinalizer callback = handle->callback();
+      void* peer = handle->peer();
+      handle->Clear();
+      (*callback)(reinterpret_cast<Dart_Handle>(handle), peer);
+    } else {
+      handle->Clear();
     }
-    Clear();
   }
 
  private: