Allow null in Dart_SetRootLibrary.

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index b9e9f554cd6c77f0784ed35103fa138a4c8e61b2..597bae21cd47885eb3d9d4c92c72c279e1a6f33d 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -5335,13 +5335,14 @@ DART_EXPORT Dart_Handle Dart_RootLibrary() {
 
 DART_EXPORT Dart_Handle Dart_SetRootLibrary(Dart_Handle library) {
   DARTSCOPE(Thread::Current());
-  const Library& lib = Api::UnwrapLibraryHandle(Z, library);
-  if (lib.IsNull()) {
-    RETURN_TYPE_ERROR(Z, library, Library);
-  }
-  Isolate* isolate = Isolate::Current();
-  isolate->object_store()->set_root_library(lib);
-  return library;
+  const Object& obj = Object::Handle(Z, Api::UnwrapHandle(library));
+  if (obj.IsNull() || obj.IsLibrary()) {
+    Library& lib = Library::Handle(Z);
+    lib ^= obj.raw();
+    T->isolate()->object_store()->set_root_library(lib);
+    return library;
+  }
+  RETURN_TYPE_ERROR(Z, library, Library);
 }