Provide API support for weak handles and post-mortem finalization.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
diff --git a/runtime/vm/dart_api_impl.h b/runtime/vm/dart_api_impl.h
index dbfd18ab9e66f9974b1e5d40e800611ae48a8cb2..0a2abe4bf42bab2ab9bddcfba6c393533e781ab7 100644
--- a/runtime/vm/dart_api_impl.h
+++ b/runtime/vm/dart_api_impl.h
@@ -13,6 +13,7 @@ namespace dart {
 class ApiState;
 class LocalHandle;
 class PersistentHandle;
+class WeakPersistentHandle;
 
 const char* CanonicalFunction(const char* func);
 
@@ -92,6 +93,11 @@ class Api : AllStatic {
   static PersistentHandle* UnwrapAsPersistentHandle(const ApiState& state,
                                                     Dart_Handle object);
 
+  // Validates and converts the passed in handle as a weak persistent handle.
+  static WeakPersistentHandle* UnwrapAsWeakPersistentHandle(
+      const ApiState& state,
+      Dart_Handle object);
+
   // Casts the internal Isolate* type to the external Dart_Isolate type.
   static Dart_Isolate CastIsolate(Isolate* isolate);